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-pattern 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)

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?

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?

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
th ere. 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.

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.