Relative clauses to postmodify a noun

 We use relative clauses to postmodify a noun - to make clear which person or thing we are talking about. In these clauses we can have the relative pronoun who, which, whose or that

·         as subject (see Clauses Sentences and Phrases)

Isn’t that the woman who lives across the road from you?
The police said the accident that happened last night was unavoidable
The newspaper reported that the tiger which killed its keeper has been put down.

WARNING:
The relative pronoun is the subject of the clause.
We do not repeat the subject:

*The woman who [she] lives across the road…
*The tiger which [it] killed its keeper …

·         as object of a clause (see Clauses, Sentences and Phrases

Have you seen those people who we met on holiday?
You shouldn’t believe everything that you read in the newspaper.
The house that we rented in London was fully furnished.
The food was definitely the thing which I enjoyed most about our holiday.

- Sometimes we use whom instead of who when the relative pronoun is the object

Have you seen those people whom we met on holiday?

- When the relative pronoun is object of its clause we sometimes leave it out:

Have you seen those people we met on holiday?
You shouldn’t believe everything you read in the newspaper.
The house we rented in London was fully furnished.
The food was definitely the thing I enjoyed most about our holiday.

WARNING:
The relative pronoun is the object of the clause.
We do not repeat the object:

Have you seen those people who we met [them] on holiday?
The house that we rented [it] in London was fully furnished.
The food was definitely the thing I enjoyed [it] most about our holiday.

·         as object of a preposition. When the relative pronoun is the object of a preposition we usually put the preposition after the verb.:

You were talking to a woman >>> Who was the woman who you were talking to?
My parents live in that house >>> That’s the house that my parents live in.
You were talking about a book. I haven’t read it. >>> I haven’t read the book which you were talking about.

- When the relative pronoun is the object of a preposition we usually leave it out:

Who was the woman you were talking to?
That’s the house my parents live in.

- Sometimes we use whom instead of who:

Who was that woman whom you were talking about.

- When we use whom or which the preposition sometimes comes at the beginning of the clause:

I haven’t read the book about which you were talking.

- We can use the possessive form, whose, in a relative clause:

I always forget that woman’s name >>> That’s the woman whose name I always forget.
I met a man whose brother works in Moscow.

Source : https://learnenglish.britishcouncil.org/en/english-grammar/clause-phrase-and-sentence/verb-patterns/relative-clauses