special cases of relative pronouns

1. which, whom, what

2.  We use “which” (not “that”) after prepositions in
formal English:
This is the room in which I was born
 In informal English, it is more common to leave
out the relative pronoun and put the preposition
after the verb:
This is the room I was born in

3.  “Which” can be used to refer to the whole of the
preceding clause:
Adriana hasn’t come to class for two weeks, which
is a bit worrying

4.  We use “who” (neither “that” nor “who”) after
prepositions in formal English:
My sister is the only person to whom I can talk
 In informal English, it is more common to leave
out the relative pronoun and put the preposition
after the verb:
My sister is the only person I can talk to

5.  You can also use “whom” in formal English to
refer to the object of the verb:
This morning I met Chris, whom I hadn’t seen for
ages

6.  We use “what” as a relative pronoun to mean “the
thing(s) that”:
She told me what she had seen

7.  “What” and a relative clause at the beginning of a
sentence gives emphasis:
What I like best about London is the parks