WHICH AND WHOM - RELATIVE PRONOUNS

1-WHICH - is used to refer to animals and things. And which can be replaced by that.
Carol doesn't like dogs which/that bark loudly.
These are the pants which/that were stolen from the store.

But, we don't always replace which for that, look:

These are the pants which/that were stolen from the store. In this example, the which-part of the sentence heps us to identify the pants the speaker is talking, without this information, the sentence wouldn't make any sense. In this case, which can be replaced by that.

She told me her name, which I can't remember right now. In this example, the first part of the sentence makes sense on its own. The which-part of the sentence gives some extra information about her name. This information could be left out. In this case, which cannot be replaced by that.

2-WHOM - is used to refer to people. In formal english, whom can be used instead of who when the relative pronoun is not followed immediately by a verb.
The woman whom/who Paul is speaking to is my mother.

Now look:
The woman whom/who John married turned out to be an good wife.
The woman who married John turned out to be an good wife.
In the second sentence, who is followed immediately by a verb. In this case whom cannot be used instead of who.

Who cannot be used immediately after prepositions. In this case, whom must be used.
He's the doctor about whom I was telling you.

We usually put the preposition after the verb and use who/that instead of whom.
Most of the people who/that I send a letter to live in Arcos.

See you latter,
Suely Vale.