Relative clauses (defining & non-defining)

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In short

A relative clause adds information about a noun, using words like who, which, that, whose, where, when. Defining clauses give essential information and take no commas; non-defining clauses add extra information and are wrapped in commas.

The rule

A relative clause comes straight after the noun it describes and tells you which one, or gives more about it.

Defining clauses identify the noun. Without them the sentence is unclear or means something else. They use no commas.

The woman who lives next door is a doctor. (which woman? this one)

Non-defining clauses give extra information that you could remove and still understand the sentence. They are separated by commas (one comma if the clause ends the sentence).

My neighbour, who is a doctor, lives next door. (we already know which neighbour)

Choose the relative pronoun by what the noun is:

Refers to	Pronoun(s)
people	who, that (defining only), whose
things / animals	which, that (defining only), whose
possession	whose
places	where
times	when
reasons	why (after the reason)

In defining clauses you can drop the pronoun when it is the object of the clause: the film (that) I saw. You cannot drop it when it is the subject: the man who called (not the man called).

In non-defining clauses use who or which, never that, and never omit the pronoun.

Which can also refer back to a whole clause: He was late, which annoyed me.

With prepositions, the formal pattern is preposition + whom/which (the person to whom I spoke); in everyday English the preposition goes to the end (the person I spoke to).

Forms & patterns

Type	Commas?	Pronouns	Omit pronoun?	that allowed?
Defining	No	who, which, that, whose, where, when	Yes, if object	Yes
Non-defining	Yes	who, which, whose, where, when	No	No

Subject vs object (defining):

Role of pronoun	Example	Can omit?
Subject	the man who called	No
Object	the film (that) I saw	Yes

Examples

English	Note
The student who asked the question got top marks.	Defining, subject pronoun - cannot be dropped.
The cake (that) she baked was delicious.	Defining, object pronoun - can be omitted.
That’s the girl whose brother plays in the band.	whose shows possession.
This is the café where we first met.	where for a place.
Do you remember the day when we moved house?	when for a time.
My uncle, who lives in Canada, is visiting us.	Non-defining - extra info, commas, no that.
The bridge, which was built in 1900, is closed.	Non-defining about a thing - use which, not that.
She passed the exam, which made everyone proud.	which refers to the whole previous clause.
He is the colleague to whom I sent the report.	Formal preposition + whom.

Common mistakes

My sister, that is a nurse, works nights. -> My sister, who is a nurse, works nights. - non-defining clauses never use that.
The man who he called me was angry. -> The man who called me was angry. - don’t add an extra subject pronoun; who is already the subject.
This is the book what I read. -> This is the book (that) I read. - use that/which, never what, in relative clauses.
The town, where I was born, is small (meaning a specific town) -> The town where I was born is small. - no commas if the clause identifies which town.
Berlin which is in Germany is huge. -> Berlin, which is in Germany, is huge. - a unique noun needs the non-defining commas.

Tips

Comma test: if you can drop the clause and still know who/what you mean, use commas (non-defining). If dropping it loses the meaning, no commas (defining).
Only drop the pronoun when something else (a new subject) follows it: the film I saw = object dropped; you can never drop who called.
that is a defining-only word. The moment you add commas, switch to who or which.
For preposition clauses, the end-position pattern (the person I spoke to) is normal in speech; save to whom for formal writing.