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.