Whom

Johnny Depp is an actor for whom acting seems easy.

After prepositions use "whom" for a person.
He's the only person to whom I can really talk.

! Sentences with "whom" can be expressed more informally without a relative pronoun and with the preposition after the verb.
He's the only person I can really talk to.

Whom is also used in formal English instead of who, when it refers to the object of the verb in the clause.
The woman whom you met was a friend of mine.

What

Which

Use "which" (not that) after prepositions.
The room in which we work is big and light.

Use which to refer to the whole previous clause.
Ex: James didn't phone, which is very surprising.