Grammar / B2 Grammar lessons and exercises / Relative clauses: Defining and non-defining
There are two types of relative clauses: defining and non-defining. In the grammar chart below, you can see the main differences between them.

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Relative pronouns are the words that introduce relative clauses. They can act as the subject or the object of the relative clause.

❗ Note:
That can be used in informal English instead of who/whom/which, but it is never used after a comma (in non-defining relative clauses) or after a preposition.
We use which/that as relative pronouns. They refer back to a noun or sentence.
We don’t use what as a relative pronoun. It cannot refer back to a noun or sentence.
❌ I gave her the letter what I had been keeping since the war.
❌ He offered to help us, what was a nice gesture.
We use what independently to mean “the thing/s that”:
It’s not very common to use prepositions before relative pronouns; we only do it in formal language.
❗ Note:
After a preposition, we can only use whom or which. (NOT who or that).
We can also use whose after a preposition:
The team signed the young Maradona, in whose skills everybody had their hopes.
The most common position of the preposition is at the end (informal/spoken English).
Relative adverbs introduce relative clauses about time (when), place (where), or reason (why).

❗ Note:
We can use preposition + which instead of a relative adverb. This structure is more formal and less common.
In non-defining relative clauses (= between commas), we can use of which/whom after quantifiers like some, any, none, all, both, several, enough, many, few.
We can also use quantifier + of whose: