DEFINING RELATIVE SENTENCES
Have a look at these examples:                                                                           

I talked to the man who gave you 
the present.
I gave you the letter which came this morning
Who gave you the present and which came this morning are "defining relative clauses". They tell us which person or thing the speaker mean ,ie, they define , give us essential information about the man and the letter. Which man ? The one who gave you a present. Which letter? the one that arrived this morning.
1. 
Defining relative clauses with 
Who, That and Which
2. Leaving out Who, That and Which
3. Defining relative clauses with Whose,Where,When and Why/That
4.
Relative 
clauses with prepositions + Which and Whom
5. Exercises

1. 
Defining relative clauses 
with who, that and which
   We use who for people and
which for things.
I talked to the man
who gave you the present.
I gave you the letter which came this morning
We can also use "That" instead of who or which in defining relative clauses. It is more informal.
I talked to the man
that gave you the present.
I gave you the letter that came this morning
Note that who, which or that replace the pronoun.
I talked to a man.
He gave you a present.
I talked to the man who gave you a present
NOT: I talked to the man who he gave you a present

2. Leaving out Who, 
That and Which
 The relative pronouns "who","that" or "which" 
can be the object or the subject  of a defining relative sentence.Compare:
Peter is the boy
who came yesterday
who is the 
subject  of "came": He came yesterday
Peter is the boy
who I saw in the restaurant yesterday
who is the 
object of "saw", not the subject : I saw him 
in the restaurant yesterday. ("I" is the subject)
When , in a defining relative sentence, the relative pronoun is the object , we often leave it out.
So we have the following possibilities for this sentence:
Peter is the boy
*whom I saw in the restaurant yesterday. (Very 
formal)
Peter is the boy who I saw in the restaurant 
yesterday (less formal than "whom")
Peter is the boy that I saw in the restaurant 
yesterday (less formal than "who")
Peter is the boy I saw in the restaurant yesterday (informal)
*whom (for people) is a relative pronoun that can be used instead of "who" when it is the object of the verb in a relative clause. It is very formal and not very common in everyday speech.
I met a woman whom I know

3. Defining relative clauses with Whose,Where,When and Why/That
WHOSE is used to talk about possession and it substitutes the possessive adjectives (his,her, my...)
I saw a girl .
Her beauty took my breath away
I saw a girl whose beauty took my breath away
They are the students.
Their teacher is in hospital with a broken leg
They are the students whose teacher is in hospital 
with a broken leg
WHERE is used for places and WHEN is used for times
The supermarket
where I normally  buy is closed 
Tell me the time when we can meet
After the word "reason" , we can use WHY or THAT in a relative clause.
The reason why/that I am angry is because you donīt work enough
NOTE: We can leave out "when" and "why/that"
Tell me the time we can meet
We can leave out "where" if we use a preposition.
The hotel 
where we stayed was near the sea
The hotel we stayed at was near the sea

4. Relative clauses with prepositions + Which and Whom
We can use a preposition before which and whom eg. in which, with whom in a defining relative clause
That the village in which he was 
born
The girl at whom I was looking was very pretty
But in everyday speech , it is more normal to put the preposition at the end of the clause and to leave out the relative pronoun.
That the village  he was born 
in
The girl  I was looking at was very pretty
Exercise 1:Leaving out the 
relative pronoun
Exercise 2: Leaving out who 
and which
Exercise 3: whose,when ,where, 
why
Exercise 4: Transformations
Exercise 5: Prepositions 
Exercise 6: Transformation with 
Prepositions