RELATIVE PRONOUN

Relative Pronouns

A relative pronoun is a pronoun that introduces a relative clause. It is called a "relative" pronoun because it "relates" to the word that it modifies. Here is an example:

• The person who phoned me last night is my teacher.

In the above example, "who":

• relates to "person", which it modifies
• introduces the relative clause "who phoned me last night"

There are five relative pronouns: who, whom, whose, which, that*
Who (subject) and whom (object) are generally only for people. Whose is for possession. Which is for things. In non-defining relative clauses, that is used for things. In defining relative clauses (clauses that are essential to the sentence and do not simply add extra information) that can be used for things and people**.

Relative pronouns can refer to singular or plural, and there is no difference between male and female.

Look at these examples showing defining and non-defining relative clauses:

	example sentences S=subject, O=object, P=possessive		notes
defining relative clauses	S	- The person who phoned me last night is my teacher. - The person that phoned me last night is my teacher.	That is preferable
		- The car which hit me was yellow. - The car that hit me was yellow.	That is preferable
	O	- The person whom I phoned last night is my teacher. - The people who I phoned last night are my teachers. - The person that I phoned last night is my teacher. - The person I phoned last night is my teacher.	Whom is correct but formal. The relative pronoun is optional.
		- The car which I drive is old. - The car that I drive is old. - The car I drive is old.	That is preferable to which. The relative pronoun is optional.
	P	- The student whose phone just rang should stand up. - Students whose parents are wealthy pay extra.
		- The police are looking for the car whose driver was masked. - The police are looking for the car of which the driver was masked.	Whose can be used with things. Of which is also possible.
non-defining relative clauses	S	- Mrs Pratt, who is very kind, is my teacher.
		- The car, which was a taxi, exploded. - The cars, which were taxis, exploded.
	O	- Mrs Pratt, whom I like very much, is my teacher. - Mrs Pratt, who I like very much, is my teacher.	Whom is correct but formal. Who is common in spoken English and informal written English.
		- The car, which I was driving at the time, suddenly caught fire.
	P	- My brother, whose phone you just heard, is a doctor.
		- The car, whose driver jumped out just before the accident, was completely destroyed. - The car, the driver of which jumped out just before the accident, was completely destroyed.	Whose can be used with things. Of which is also possible.

*Not all grammar sources count "that" as a relative pronoun.
**Some people claim that even in defining relative clauses we cannot use "that" for people but must use "who/whom". There is no good reason for such a claim; there is a long history of "that" for people in defining relative clauses from Chaucer, Shakespeare and the Authorized Version of The Bible to Fowler's and Churchill.
SOURCE :  http://www.englishclub.com/grammar/pronouns-relative.htm