Relative clauses: defining & non-defining
who/which/that/whose/where/when and commas.
A relative clause modifies a noun by giving more information about it, introduced by a relative pronoun (who, whom, which, whose, that) or relative adverb (where, when). Defining relative clauses identify which person or thing we mean — they are essential to the meaning and are NOT separated from the main clause by commas; 'that' can replace 'who/which', and the pronoun may be omitted when it functions as the object of the clause (e.g., 'The book I borrowed was great'). Non-defining relative clauses add extra, non-essential information about a noun that is already identified; they ARE separated by commas, 'that' cannot be used, and the relative pronoun can never be omitted (e.g., 'My sister, who lives in London, is a doctor'). The relative pronoun 'whose' shows possession for both people and things. 'Where' replaces 'in/at which' for places, and 'when' replaces 'in/at which' for times. For example: 'The school where I studied, which was built in 1920, still stands today' — the second clause is non-defining because 'the school' is already identified.
Key terms
| Pronoun / Adverb | Refers to | Role | Example |
|---|---|---|---|
| who | people | subject or object | The teacher who called is kind. / The man who(m) I met was friendly. |
| whom | people (formal) | object only | The professor whom we consulted gave great advice. |
| which | things | subject or object | The book which I bought was excellent. |
| that | people or things (DEFINING only) | subject or object | The student that passed was my friend. |
| whose | people & things (possession) | possessive determiner | The scientist whose research changed medicine won a prize. |
| where | places | replaces 'in/at which' | The city where I was born is Baku. |
| when | times | replaces 'in/at which' | I remember the summer when we first met. |
Note: 'That' can NEVER be used in a non-defining relative clause.
| Feature | Defining | Non-defining |
|---|---|---|
| Commas | NO commas | YES — commas required |
| Can use 'that'? | Yes | No — never |
| Can omit pronoun? | Yes, if it is the object | No — never |
| Head noun | Not yet identified (essential) | Already identified (extra info) |
| Remove the clause? | Meaning changes / becomes unclear | Meaning stays clear |
| Example | The student that passed is my friend. | My brother, who lives in Berlin, is an engineer. |
Key test: can you remove the clause without losing the identity of the noun? If yes → non-defining (commas). If no → defining (no commas).
| Position of pronoun | Can omit? | Example (omitted form) |
|---|---|---|
| Object of the relative clause (defining only) | YES | The film we watched was brilliant. (= that/which we watched) |
| Subject of the relative clause | NO | The singer who sang the song was famous. ('who' cannot be removed) |
| Any position in non-defining clause | NO | My friend, who lives in Paris, is a chef. ('who' cannot be removed) |
| Possessive (whose) | NO | The artist whose paintings you admire... ('whose' cannot be removed) |
Simple check: if you can find another subject for the relative clause verb, the pronoun is the object and can be omitted.
- 1Step 1 — Identify the head noun: Look at the noun the relative clause describes. Is it already uniquely identified? Example: 'London' (unique proper noun) vs 'a city' (unidentified).
- 2Step 2 — Ask: is the clause essential?: Try removing the clause. 'London is one of the world's great cities.' → meaning preserved. 'The bridge is still in use.' → which bridge? Meaning lost.
- 3Step 3 — Choose clause type: Meaning lost without clause → DEFINING (no commas, 'that' allowed, pronoun omissible if object). Meaning preserved → NON-DEFINING (commas required, 'that' forbidden, pronoun never omissible).
- 4Step 4 — Choose the correct pronoun: Person as subject → 'who'. Person as object (formal) → 'whom'. Thing → 'which' or 'that' (defining only). Possession → 'whose'. Place → 'where'. Time → 'when'.
- 5Step 5 — Write the sentence: Defining: 'The bridge that was built in 1887 is still in use.' Non-defining: 'London, where the 2012 Olympics were held, is one of the world's great cities.'
'That' in a non-defining clause is a very common error. 'My laptop, that I bought last year, stopped working.' is WRONG. Non-defining clauses (with commas) require 'who' for people and 'which' for things — 'that' is forbidden. Correct: 'My laptop, which I bought last year, stopped working.'
Do NOT confuse 'whose' (possessive relative pronoun) with 'who's' (= who is). 'The scientist who's research changed medicine...' is wrong — 'who is research' makes no sense. Always use 'whose' for possession: 'the scientist whose research changed medicine'.
Double object error: after a relative pronoun, do NOT repeat the object with a personal pronoun. 'The essay which I worked on it...' is wrong — 'which' already fills the object slot. Remove 'it': 'The essay which I worked on for three weeks received the highest grade.'
Use 'where' (not 'which') for places without a preposition. 'The hotel which we wanted to stay was fully booked' is incomplete — you need 'in which'. The simple solution: 'The hotel where we wanted to stay was fully booked.' Similarly, 'when' replaces 'at/in which' for time expressions: 'the day when we graduated'.
Commas signal meaning. 'Teachers who care about students achieve great results' (defining) = only those teachers who care succeed — implying some do not care. 'Teachers, who care about students, achieve great results' (non-defining) = a statement about all teachers as a group. A comma changes the logic of the sentence.
Rules
- 1Defining relative clauses identify the noun and are NOT separated by commas; 'that' can replace 'who/which' and the object pronoun can be omitted.
- 2Non-defining relative clauses add extra information and ARE separated by commas; 'that' cannot be used and the pronoun cannot be omitted.
- 3Use 'who' for people (subject), 'whom' for people (object/formal), 'which' for things, and 'whose' for possession (people and things).
- 4Use 'where' instead of 'in/at which' for places, and 'when' instead of 'in/at which' for times.
- 5In a defining clause, the relative pronoun can be omitted only when it is the object of the relative clause, never when it is the subject.
Practice
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