Relative clauses let writers identify, describe and add information about people, things, places, times and ideas without starting a new sentence every time. They are central to sentence expansion, precise noun phrases, comprehension, editing, academic definition and descriptive writing. Yet they are also a common source of errors involving who, whom, which, that, whose, where, commas, omitted pronouns and subject-verb agreement.
This SETC guide explains defining and non-defining relative clauses, relative pronouns, relative adverbs, omission, reduced relatives, prepositions, agreement, punctuation, advanced structures and the transfer of relative-clause control into real writing.
The central idea is that a relative clause is attached to an antecedent and performs a job inside a larger noun phrase. Before choosing who, which or that, identify what the clause modifies and whether the information identifies the noun or merely adds extra information.
What is a relative clause?
A relative clause is a dependent clause that modifies a noun or noun phrase. In The student who answered first received a prize, who answered first modifies the student. The clause cannot normally stand alone as an independent sentence.
Relative clauses allow information to be integrated rather than repeated: I met a student. The student won the competition. becomes I met a student who won the competition.
The antecedent
The antecedent is the noun phrase that the relative expression refers to. In The book that I borrowed is on the table, the book is the antecedent of that. In The teacher whose class won spoke first, the teacher is the antecedent of whose.
Clear antecedents matter because a relative clause can become ambiguous if it appears after more than one possible noun.
Defining relative clauses
A defining relative clause identifies which person or thing is meant: The student who borrowed the laptop should return it. The clause who borrowed the laptop distinguishes that student from others.
Defining clauses are essential to the reference. They are not normally separated by commas.
Non-defining relative clauses
A non-defining relative clause adds extra information about an already identifiable noun: Maren, who borrowed the laptop yesterday, has returned it. Maren is already identified by name; the relative clause adds supplementary detail.
Non-defining clauses are normally set off by commas, and that is generally not used as the relative pronoun in standard formal English.
Who
Who commonly refers to people and can function as subject: The student who answered first was correct. Who is the subject of answered.
In defining and non-defining clauses, who is widely used for people. In everyday English it also often replaces whom in object position.
Whom
Whom is the traditional object form: The student whom I met was helpful. In modern English, who is more common in many object contexts: The student who I met was helpful.
Whom remains frequent after fronted prepositions and in formal styles: The colleague to whom I wrote replied immediately.
Which
Which commonly refers to things and animals in many contexts: The book which I borrowed is useful. The new system, which was introduced last month, has reduced delays.
Which is standard in non-defining clauses. In defining clauses, both which and that are possible in many varieties and styles.
That
That can introduce defining relative clauses referring to people or things: The student that answered first; the book that I borrowed. Many writers prefer who for people, but that is grammatical in many defining contexts.
That is not normally used in non-defining clauses: My laptop, which is five years old, still works, not My laptop, that is five years old, still works.
Whose
Whose expresses possession: the student whose notebook is missing; the company whose policy changed. It can refer to people and, in standard modern usage, organisations and things.
Whose is often more natural than awkward alternatives such as of which.
Where
Where can introduce relative clauses about place: The library where we studied is closing. This can often be paraphrased as the library in which we studied.
Where should refer to a place or place-like context. Do not use where merely as a general substitute for which.
When
When can introduce relative clauses about time: I remember the day when we first met. The year when the policy changed was difficult.
In many contexts when can be omitted: I remember the day we first met.
Why
Why can appear after reason: The reason why the meeting was cancelled is unclear. The reason the meeting was cancelled is also standard.
Avoid the redundant construction the reason is because in highly formal writing when a cleaner structure is available, though it is common in speech.
Subject relative pronouns
When the relative pronoun is the subject of the relative clause, it cannot normally be omitted: The student who called me was polite. Who is the subject of called.
Writing The student called me was polite would create a different structure or ambiguity.
Object relative pronouns
When the relative pronoun is the object in a defining clause, it can often be omitted: The book that I borrowed is useful. The book which I borrowed is useful. The book I borrowed is useful.
Omission is not normally possible in the same way in non-defining clauses.
How to tell subject from object
Look immediately after the relative pronoun. If a subject is already present, the relative pronoun may be the object: the book that I borrowed. I is the subject of borrowed, so that is object. If no other subject appears, the relative pronoun is likely subject: the book that fell.
This structural test is more reliable than guessing from meaning.
Relative-clause agreement
The verb inside a relative clause agrees with the antecedent represented by the relative pronoun: the student who works; the students who work. The surface word who does not have a fixed singular or plural value.
This matters in structures such as one of the students who are presenting, where who usually refers to students.
One of the students who…
In She is one of the students who are presenting, the relative clause normally modifies students, so are is plural. The meaning is that several students are presenting, and she is one of them.
Compare She is the only one of the students who is presenting, where the intended antecedent can be the only one, making singular agreement natural.
Preposition at the end
Everyday English often leaves the preposition at the end of the relative clause: The person who I spoke to was helpful. The topic that we talked about was difficult.
This is called preposition stranding and is standard in many contexts.
Fronted prepositions
Formal English may place the preposition before whom or which: The person to whom I spoke was helpful. The topic about which we talked was difficult.
That cannot normally follow a fronted preposition: the topic about that we talked is non-standard.
Relative clauses with quantifiers
Advanced non-defining clauses can include expressions such as many of whom, some of which, none of whom and both of which: The class had thirty students, many of whom travelled by bus. We tested three methods, two of which failed.
These structures compress relationships that would otherwise require separate sentences.
Which referring to a whole clause
Which can refer to an entire previous clause in a non-defining structure: The meeting was cancelled, which surprised everyone. Which refers to the fact that the meeting was cancelled.
This is different from which referring to a single noun.
What
What can mean the thing that or the things that: What she said surprised me. It does not normally take a separate antecedent. We do not say the thing what she said in standard English.
This distinction is useful because learners sometimes treat what as interchangeable with that.
Reduced relative clauses
Relative clauses can sometimes be reduced to participial phrases. Students who live nearby can become students living nearby. The documents that were submitted yesterday can become the documents submitted yesterday.
Reduction is most natural when the meaning remains clear and the relative pronoun is subject.
Present-participle reductions
An active relative clause can often reduce to -ing: people who work remotely → people working remotely; a machine that produces heat → a machine producing heat.
The reduced phrase still modifies the noun and inherits the active relationship.
Past-participle reductions
Passive relative clauses can often reduce to a past participle: the report that was published yesterday → the report published yesterday; the students who were selected → the students selected.
This compression is common in academic and formal prose.
Infinitival relative structures
Some noun phrases use to-infinitives with relative meaning: the first student to finish, the best method to use, a place to study. These structures do not contain an overt relative pronoun but perform similar modifying work.
They are especially common after ordinals, superlatives and indefinite nouns.
Defining versus non-defining meaning changes
Compare My brother who lives in Perth is visiting and My brother, who lives in Perth, is visiting. Without commas, the sentence can imply that I have more than one brother and the clause identifies which one. With commas, the noun my brother is already identifiable and the clause adds extra information.
Punctuation can therefore change meaning, not merely style.
Proper names
Proper names are already identifiable, so relative clauses after them are usually non-defining: Iona, who lives nearby, arrived first.
A defining clause can still appear with a name in special contrastive contexts, but ordinary school writing should recognise the non-defining pattern.
Non-defining clauses and that
Standard formal English normally uses who or which, not that, after a comma: Maren, who won the prize, thanked her teacher. The book, which was expensive, was useful.
This is a common editing point.
Comma placement
A non-defining clause inserted inside a sentence normally needs a comma at both ends: The report, which was published yesterday, contains new data. Forgetting the second comma can make the structure difficult to parse.
If the clause comes at the end, one opening comma is sufficient: The report contains new data, which surprised the team.
Ambiguous attachment
Consider: I spoke to the teacher of the student who won the prize. Who won the prize—the student or the teacher? Grammar and proximity suggest student, but the intended meaning may differ.
Rewrite when attachment is unclear: I spoke to the teacher whose student won the prize, or I spoke to the teacher, who had won the prize.
Relative clauses and information density
Relative clauses allow writers to pack information into noun phrases, but too many nested clauses create processing difficulty: The student who borrowed the book that the teacher who joined last year recommended… may be grammatical but exhausting.
Good writing balances integration with readability.
Relative clauses in definitions
Definitions often use relative clauses: A photosynthesising organism is an organism that converts light energy into chemical energy. A hypothesis is a proposition that can be tested against evidence.
The clause identifies a defining property, which makes relative grammar useful across subjects.
Relative clauses in descriptions
Descriptive writing uses relative clauses to add selective detail: The house, which stood at the end of a narrow lane, had no lights on. A man who carried a red umbrella waited outside.
Defining and non-defining choices control whether the detail identifies or merely enriches the referent.
Relative clauses in arguments
Argumentative writing can use relative clauses to restrict claims: Policies that reduce barriers can improve access. The clause limits the claim to policies with a particular property.
Removing the clause would broaden the claim and potentially make it less accurate.
Relative clauses in comprehension
Readers must identify exactly which noun a relative clause modifies. Long passages may separate the antecedent from the relative verb or embed one relative clause inside another.
A useful strategy is to bracket the clause and draw an arrow to the antecedent.
Common learner errors
- The student which called me → the student who called me
- The book who I borrowed → the book that/which I borrowed
- My laptop, that is old, still works → which
- The student who I met her → remove her because who is already object
- The person to who I spoke → to whom / who I spoke to
- The students who works → who work
- The book that I borrowed it → remove it
- The place which I studied → the place where I studied / at which I studied
- The teacher, who class won → whose class won
- The reason why because… → choose a cleaner reason/why/because structure
Resumptive pronouns
A common learner pattern is The book that I borrowed it was useful. The pronoun it repeats the object position already represented by that. Standard English normally omits it: The book that I borrowed was useful.
Similarly, The student who she won is non-standard because who already fills the subject position.
A clause-building routine
Start with two sentences. Identify the repeated noun. Decide which sentence should become the relative clause. Choose the relative expression by antecedent type and grammatical role. Then decide whether the clause is defining or non-defining and punctuate accordingly.
Example: Iona met a researcher. The researcher studies memory. → Iona met a researcher who studies memory.
Worked example: object omission
I read the article that Maren recommended can become I read the article Maren recommended because that is the object of recommended and the clause is defining.
The shortened form is natural and common.
Worked example: non-defining
Leonie’s laptop, which she bought five years ago, still works. The laptop is already identified by possession; the clause adds supplementary information and is enclosed by commas.
Worked example: preposition
The colleague who I wrote to replied. More formal: The colleague to whom I wrote replied. Both are standard; register differs.
Worked example: whole-clause which
The school extended the deadline, which reduced student stress. Which refers to the entire decision to extend the deadline, not merely to deadline.
Primary and PSLE application
Primary learners should begin by joining pairs of sentences with who, which and that. Then introduce whose and where. Meaning should remain visible: who for people, which/that for things, whose for possession.
PSLE synthesis and transformation can draw directly on relative-clause control, while composition benefits from sentence expansion without endless and-chains.
Secondary and SEC application
Secondary learners need defining versus non-defining punctuation, omission, prepositions and agreement. They should also learn to reduce relative clauses where appropriate.
Editing practice should include sentences where commas change meaning, not only pronoun substitution.
Advanced academic writing
Academic prose uses relative clauses to define concepts, restrict claims and compress information. Reduced relatives are especially common: participants recruited from three schools; results obtained after calibration; factors affecting retention.
Compression is useful only when the logical relationship remains clear.
A diagnostic practice set
- The student who called me was polite.
- The student who I called was polite.
- The student I called was polite.
- Maren, who called me, was polite.
- The book, that I borrowed, was useful.
- The book which I borrowed was useful.
- The colleague to whom I wrote replied.
- The colleague who I wrote to replied.
- The students who work hardest improve quickly.
- The report published yesterday contains new data.
Items 1 to 4 are standard. Item 5 should normally use which in the non-defining clause. Items 6 to 10 are standard and illustrate object relatives, preposition placement, agreement and reduction.
A paragraph editing challenge
Draft: Iona spoke to a researcher which studies memory, and the article that the researcher recommended it was useful. The university, that employs the researcher is nearby, and the laboratory which she works has recently expanded.
Revision: Iona spoke to a researcher who studies memory, and the article that the researcher recommended was useful. The university, which employs the researcher, is nearby, and the laboratory where she works has recently expanded.
How to practise for durable control
Begin with sentence joining, then classification of defining and non-defining clauses, then omission tests, preposition variants, agreement and reduced relatives. Finally, edit authentic paragraphs and use relative clauses in original writing.
The transfer test is whether learners can choose relative structures while focusing on ideas rather than on a labelled grammar exercise.
Teacher diagnosis
If a learner misuses a relative pronoun, check antecedent type and clause role. If punctuation is wrong, check whether the learner understands identification versus extra information. If resumptive pronouns appear, check whether they understand that the relative expression already fills a grammatical slot.
Different surface errors have different causes.
Frequently asked questions
Can I use that for people?
Yes, in many defining clauses, though who is often preferred for people. That is not normally used in non-defining clauses.
Can I omit the relative pronoun?
Often when it is the object of a defining clause. Not normally when it is the subject.
What is the difference between defining and non-defining clauses?
Defining clauses identify which noun is meant; non-defining clauses add extra information about an already identifiable noun.
Do non-defining clauses need commas?
Yes, they are normally separated by commas from the main clause.
Can whose refer to things?
Yes. Modern standard English allows whose with organisations and things where possession or association is intended.
Why is agreement sometimes plural after who?
Who inherits number from its antecedent: student who works, students who work.
Research and reference points
This guide aligns with Cambridge Grammar and British Council descriptions of defining and non-defining relatives, pronoun roles, omission and preposition placement. SETC extends these structures into comprehension, sentence building and writing transfer.
Continue through SETC
Continue with How English Works: Relative Clauses, Pronoun-Antecedent Agreement, Sentence Boundaries and English Grammar and Sentence Control.
The mastery target is precise modification: identify the antecedent, choose the relative expression, control clause role and agreement, and use punctuation to tell the reader whether the information identifies or merely adds detail.
Relative clauses and noun-phrase architecture
A relative clause is one kind of postmodifier inside a noun phrase. In the phrase the student who answered first, the head noun is student and the relative clause narrows or describes that head. Other postmodifiers include prepositional phrases and participial phrases: the student in the front row, the student carrying the red folder. Seeing these as one family helps advanced learners understand how English builds detailed noun phrases around a head.
This matters in editing because a long noun phrase can separate the head from the main verb. The group of students who were selected for the final round is meeting today. The main subject head is group, so the main verb is singular, even though were inside the relative clause agrees with students.
Embedded relative clauses
Relative clauses can appear inside larger relative structures: The report that describes the experiment which failed was revised. Here which failed modifies experiment, while that describes the experiment which failed modifies report. These layers are grammatical but increase processing load.
When a sentence contains two or more embedded relatives, ask whether separate sentences would be clearer. Complexity is useful when it preserves a tight relationship; it is harmful when the reader must repeatedly backtrack to find the antecedent.
Attachment and the nearest-noun temptation
Readers often attach a relative clause to the nearest plausible noun, but intended meaning can override proximity only if the syntax makes that intention clear. The picture of the student who won the prize normally means the student won. If the picture itself won a photography prize, rewrite: The picture, which won the prize, shows a student.
Good writers anticipate default attachment and place the relative clause immediately after the noun it modifies whenever possible.
Relative clauses with indefinite pronouns
Relative clauses can modify indefinite antecedents: anyone who needs help, someone who knows the answer, nothing that matters, everything that changed. These structures are useful because the relative clause defines the otherwise broad antecedent.
Who is natural with human antecedents such as anyone and someone. That is common with everything, nothing, something and superlative or all-type meanings: everything that happened, the best idea that we found. Usage varies, so learners should notice authentic patterns rather than convert preferences into absolute rules.
Relative clauses after superlatives and ordinals
Relative clauses frequently follow superlatives and ordinals: the first student who finished, the best solution that we found, the only method that worked. In many traditional styles, that is preferred after superlatives and words such as all, everything or only, but which can appear in some varieties. The larger principle is still defining modification.
Reduced infinitival forms are also common: the first student to finish, the best solution to use, the only method to work in that environment. The exact reduction depends on meaning and idiom.
Relative clauses and punctuation scope
Commas around non-defining clauses tell the reader where the supplementary information begins and ends. Compare The students who had finished left early with The students, who had finished, left early. The first may mean only the finished students left. The second presents all the relevant students as having finished, then adds that information parenthetically.
Punctuation therefore changes the set being referred to. In formal writing, this can affect factual meaning, not just rhythm.
Restrictive meaning without an overt relative pronoun
English often omits the object relative pronoun so smoothly that learners fail to notice the clause: the book I borrowed, the person we met, the question I answered. The missing pronoun still has a grammatical role inside the relative clause even though it is not pronounced.
A useful analysis is to restore the pronoun mentally: the book that I borrowed. If the restored pronoun would be subject, omission is usually not possible: the book that fell cannot normally become the book fell inside the same noun phrase.
Relative adverbs versus preposition + which
Place, time and reason can often be expressed in two ways: the house where I grew up / the house in which I grew up; the year when we met / the year in which we met; the reason why she left / the reason for which she left. The relative-adverb forms are usually more natural in everyday English.
Formal writing may prefer preposition + which where the relationship needs to be explicit, but unnecessarily formal versions can sound stiff. Match the construction to genre.
Reduced relatives in headlines and labels
Headlines, labels and technical descriptions often use reduced relatives because space is limited: Students selected for final round; Data collected in 2025; Devices operating above 80°C. These phrases omit forms of be or relative pronouns but remain interpretable because the noun-modifier relationship is clear.
Students should not copy headline compression blindly into ordinary prose. What is acceptable in a label may become a fragment if placed as a full sentence without a main clause.
Relative clauses in paraphrase
Paraphrasing often requires changing relative structure without changing meaning. The students who were selected for the programme can become the students selected for the programme. The device that measures temperature can become the temperature-measuring device in some contexts. The people who live nearby can become local residents.
Each paraphrase changes grammatical packaging. Writers must preserve the same restriction: if the original refers only to a subset, the paraphrase must not accidentally broaden the claim to the whole group.
Relative clauses and source attribution
Academic writing often uses relative clauses to locate information precisely: The study, which included 240 participants, found a small effect. The theory that the authors proposed was later revised. The first clause is supplementary; the second identifies which theory is being discussed.
Choosing defining versus non-defining structure can therefore affect how many studies, theories or authors the sentence appears to refer to.
Relative-clause editing pass
During a focused edit, underline who, whom, whose, which, that, where and when. Draw an arrow to each antecedent. Mark S if the relative expression is subject and O if it is object. Then decide whether the clause identifies or merely adds information. Finally, check agreement, pronoun omission and commas.
This structural pass catches several errors at once: wrong relative word, resumptive pronoun, comma misuse and incorrect verb number.
A second diagnostic set
- The student who won the prize thanked her teacher.
- The student whom I congratulated thanked her teacher.
- The student I congratulated thanked her teacher.
- Iona, whose essay won the prize, thanked her teacher.
- The essay, which was published online, attracted attention.
- The essay that was published online attracted attention.
- The office where we met has moved.
- The office in which we met has moved.
- The files submitted yesterday are being reviewed.
- The decision was delayed, which frustrated everyone.
All ten are grammatical. The task is to explain the different information structures: object omission in item 3, possession in item 4, non-defining versus defining contrast in items 5 and 6, alternative place relatives in items 7 and 8, reduction in item 9, and whole-clause which in item 10.
Final mastery check
Relative-clause mastery means the learner can identify antecedents, distinguish defining from non-defining information, select who/whom/which/that/whose/where accurately, omit object relatives when appropriate, place prepositions naturally, control agreement and reduce clauses without changing scope.
The final test is whether those choices survive real writing. A learner should be able to define a concept, describe a person, restrict a claim, combine sentences and compress information while keeping the reader certain about what each modifier belongs to.
Editing long noun phrases with relative clauses
Long noun phrases can hide both the antecedent and the main sentence spine. Consider The set of recommendations that the committee published after the review, which lasted three months, has been revised. The head of the main subject is set, so the main verb is has. That the committee published after the review modifies recommendations. Which lasted three months most naturally modifies review. Mapping each clause to its antecedent prevents agreement and attachment errors.
When several modifiers compete, reduction may help: The committee’s recommendations, published after a three-month review, have been revised. The new sentence changes the head noun from set to recommendations, so the main verb becomes have. Editing relative clauses can therefore alter agreement elsewhere in the sentence; every structural revision should trigger a new sentence-spine check.
Relative-clause ambiguity lab
Take the sentence I spoke to the assistant of the teacher who organised the event. The nearest-noun reading makes who refer to teacher, but the intended organiser might be the assistant. Two repairs are possible: I spoke to the teacher’s assistant, who organised the event, or I spoke to the assistant whose teacher organised the event. The choice depends on meaning, and punctuation participates in that meaning.
Now compare The parents of students who arrived late waited outside with The parents, whose students arrived late, waited outside. The first defining clause may select only the students who arrived late, leaving open which parents are meant. The second presents the parents as already identified and adds information about their students. Relative grammar can change the size of the group being described.
Relative clauses in synthesis and transformation
Sentence-combining tasks often require preserving every important meaning while removing repetition. Leonie borrowed a reference book. The reference book contained model explanations. It had been recommended by her tutor. can become Leonie borrowed a reference book that contained model explanations and had been recommended by her tutor. Both relative meanings attach to the same noun, so the repeated subject does not need to be restated.
A stronger learner also checks whether a non-defining version would change the implication. Leonie borrowed the reference book, which contained model explanations, suggests that the book is already identifiable. Article choice and relative-clause type therefore interact.
Relative clauses as precision tools
In analytical writing, relative clauses can prevent sweeping claims. Students who receive timely feedback may revise more effectively is narrower than Students may revise more effectively. Technologies that reduce repetitive work can improve efficiency is narrower than Technologies can improve efficiency. The modifier defines the population or category to which the claim applies.
During final revision, ask whether each defining relative clause genuinely identifies a subset and whether removing it would accidentally broaden the claim. This turns relative-clause editing into a reasoning check as well as a grammar check.