Object Adjective Clauses

Like subject adjective clauses, object adjective clauses also have some special characteristics:

1. Object Relative Pronouns: People
For people, who(m) or that can be used; both are common. However, whom is used mostly for very formal speaking or writing. For other uses (including most speaking), who or that is used. The only
difference between who in subject- and in object adjective clauses is in word order:

That's the person who was talking to us. (subject pattern)

That's the person who we were talking to. (object pattern)

2. Object Relative Pronouns: Things
For things, that or which can be used; both are common:

Is this the book that / which you wanted?

When do I have to repay the money that / which I borrowed?

3. Omitting Relative Pronouns
In object-pattern adjective clauses, relative pronouns are often omitted, particularly in speaking:

That's the person who(m) / that we were talking to. --->That's the person we were talking to.
The person who(m) / that we met at Jack's party called me. ---> The person we met at Jack's party called me.

Is this the book that / which you wanted? ---> Is this the book you wanted?

When do I have to repay the money that / which I borrowed? ---> When do I have to repay the money I borrowed?
4. Adjective Clauses as Objects of Prepositions: People

Relative pronouns for people can be the objects of prepositions. In this situation, the word order is different in formal and "everyday" English. Also, only whom (not who or that) can be used to refer to people in the formal version:

The woman is right over there. You were talking about the woman. --->

formal: The woman about whom you were talking is right over there.
"everyday": The woman who / that you were talking about is right over there. (or The woman you were talking about is right over there.)

wrong: *The woman about who you were talking is right over there.
wrong: *The woman about that you were talking is right over there.

5. Adjective Clauses as Objects of Prepositions: Things
Relative pronouns for things can also be the objects of prepositions. In this situation, the word order is also different in formal and "everyday" English. Also, only which (not that) can be used to refer to things in the formal version:

The book is rather expensive. He referred to the book. --->

formal: The book to which he referred is rather expensive.
"everyday": The book that / which he referred to is rather expensive. (or The book he referred to is rather expensive.)

wrong: *The book to that he referred is rather expensive.

taken from:
http://www.eslcafe.com/grammar/adjective_clauses05.html