Relative clauses tell something about a noun which we call the antecedent.
 
     That man is Kevin. He is talking to your father
     = The man    who is talking to your father   is Kevin
    antecedent   └─┼───relative clause───┘
                    relative connector                              
 
The man is Kevin, OK, but, who is Kevin? Well, he is the person who is talking to your father. So a relative clause gives us more information about the antecedent (the man).

      I’ve got a book. It’s very interesting
      = I’ve got a book which is very interesting
 
When the antecedent is a person, we use the relative pronoun WHO. When it is a thing, we use WHICH.
 
The relative pronoun does two things: it is a connector and it can be the subject or the object of the relative clause (just the same function as the antecedent would do itself).

WHO and WHICH may both be replaced by THAT
      The person who/that comes here first will have it
      The house which/that I want is near here
 
When there is possession (for both people or things) we use WHOSE.
      The house whose windows are broken is Tony’s
      I met a girl whose name was Molly
  
WHAT
It means "the thing that", so the antecedent (thing) is included inside the connector
      I told him what he wanted to know (=the thing that he wanted to know)
      I've got what you need
 
Relative clauses of time, place and reason

When the antecedent is a word of time we use WHEN, when it is a word of place we use WHERE and when the antecedent is the word "the reason" we use WHY. In these three cases the antecedent can be removed.
 
      Monday is (the day) when I don’t work
      This is (the place) where I grew up
      That’s (the reason) why I hate her

WHEN CAN WE SUPPRESS THE CONNECTOR?

When talking, we often suppress the connector, but we can't always do that.

RELATIVE PRONOUN AS OBJECT
In this case, it is very often left out in informal English
 
      I work with a man that you know
      I work with a man you know
 
      The whisky that you drank at the party is very expensive
      The whisky you drank at the party is very expensive
 
      The girl you met yesterday is my sister
      Is this the tape you are looking for?

Compare:

- This is the book which I bought yesterday  (formal)
- This is the book that I bought yesterday  (informal)
- This is the book I bought yesterday  (more informal)
 
RELATIVE PRONOUN AS SUBJECT
In this case we can't leave the connector out because if you do, the sentence has no subject
 
      She’s a person who can do anything   (not: She's a person can do anything)
      Everything that happened was your fault
      He’s the man who lives next door
      I have a cat which bit a dog

In English, every verb needs a subject (exept imperatives). Compare:

- The perfume she likes is Channel     (The perfume is / She likes)
- The man you saw yesterday lives near here      (the man lives / you saw)
- He is the boy who broke your window      (He is / who broke)
- That is the house that looks scary      (That is / that looks: the 1st "that" is a demonstrative, the 2nd "that" is a relative connector equivalent to "which")

NON-DEFINING RELATIVE CLAUSES

All the sentences we have seen are defining a previous word, but we can also use a relative clause not to define, but to simply add more information. In this case, the relative clause must be surrounded by commas, and the connector can never be eliminated or substituted using THAT:

defining:
- The man who is wearing a hat is John (which man? the one wearing a hat)

non-defining, extending:
- The house with a garden, which was where I lived, was the most beautiful house of all (the house was the most beautiful AND the place where I lived)
- Ky, who had been walking all day, was very tired (Ky was very tired BECAUSE he had been walking all day)

A special situation is when the antecedent is not just a word, but the complete sentence. In that case we put the relative clause at the end, after a comma, and we use the connector WHICH:

- All the forest was on fire, which was terrible. (what was terrible? the fact that "all the forest was on fire")
- She comes to visit every day, which is really nice.