Wah How to use relative pronouns (Who, Whom, Which, That).

Who, Whom, Which, That are the relative pronouns.
A relative pronoun is a pronoun that introduces a relative clause.

1. Who (as subject)
We use who referring to people.
    The person who went out with me last night is my old friend.
    Annie is the girl who get this job.
    Martin, who is an English teacher, comes from New York.
    Julio, who is very kind, is my colleague.

2. Whom (as object)
We use whom referring to people.
    The person whom I spoke to was my father.
    James, whom I went to school with.
    Annie, whom I like so much, is my English teacher.
    This is Helen, whom you met at the party last week.
    The man whom you told me about went to Rome last week.

Note: But we usually use who instead of whom.
    The person who I spoke to was my father.
    This is Helen, who you met at the party last week.

3. Which (as subject or object)
We use which referring to things.

As subject
    This is a song which is popular now.
    Our dog, which is named John, barks at anyone.
    Take the book which is lying on that table.

As object
    The car, which I bought last month, was stolen.
    The camera which you bought is better than mine.
    The book which you lent me is very interesting.

4. That (as subject or object)
We use that referring to people or things.

As subject
    The lady that is speaking to that boy is his teacher.
    He's the man that was here last week.
    This is a song that is popular now.
    She was given a problem that was impossible to solve.

As object
    The person that I spoke to was my father.
    The man that you told me about went to Rome last week.
    The car that I bought last month was stolen.
    The camera that you bought is better than mine.

Note: That never follows a comma. Look at these sentences.
    The car, which I bought last month, was stolen. 
    The car that I bought last month was stolen.
    The car, that I bought last month, was stolen. is incorrect.

                                      Erin Karpluk, who is an actress, is very beautiful.