Compare the two groups of sentences :- 
(a) Rohit is a good boy. Rohit always comes first. Rohit obeys his teachers. Everybody praises Rohit. 
(b) Rohit is a good boy. He always comes first. He obeys his teachers. Everybody praises him. 

You will note that the sentences in the second group sound better. It is easier and nicer to say he or him than to repeat the noun Rohit every time. He or him has been used in place of noun Rohit. He or him is a  Pronoun. Pronoun means For-a-Noun.

Definition of Pronoun  : A Pronoun is a word which is used instead of a Noun. Examples : she, he, him, her, I, we, they, myself, this, that, those, each, every 

Kinds of Pronoun :

Pronouns are of many kinds. Pronouns are classified as follows :-

(a) Personal : - I, we, they, you, he, she, it.
(b) Possessive :- mine, your, ours, hers
(c) Relative :- who, whose, whom, which, that, what.
(d) Interrogative :- who ?, whose?, whom?, which?, what?
(e) Demonstrative :- this, that, these, those
(f) Reflexive and Emphatic :- myself, himself, herself, itself, yourself, yourselves, themselves.
(g) Indefinite :- one, any, anyone, anything, anybody, somebody, all, none. 
(h) Distributive :- each, every, either, neither.
(i) Reciporcal :- each other, one another
A. Personal Pronoun :- A Personal Pronoun is used in place of a Noun. 
For example :- 1. I love my football coach, he trains me so well.
2. You reached the destination before us

  Singular Plural Singular Plural
First Person I we me us
Second Person you you you you
Third Person he, she, it they him, her them

B. Possessive Pronoun :- A Pronoun that expresses 'possession' is called a Possesive Pronoun.
For example :- 1. This pen is his. 2. This umbrella is mine. 

  Singular Plural
First person mine ours
Second Person yours yours
Third Person his, hers, its theirs

Note the difference between the possessive adjectives and the possessive pronouns through the following examples

Possessive Pronoun Possessive Adjectives
This pen is his.  This is his pen. 
Is this book yours? Is this your book ?
Are these bottles theirs? Are these their bottles ?

Possessive Pronoun stands on its own and is not followed by a Noun.
Possessive Adjective is followed by a Noun.

C. Relative Pronoun : A Pronoun that describes the person, place or the thing you are talking about or specifying which one you mean is called a 'Relative Pronoun.'
For example :-
We found the papers. The papers were on the shelf. 
We found the papers that were on the shelf. 

In the given examples, 'that' is used in place of the nouns, 'the papers'. Thus that are relative pronouns directly related to the noun of another clause.
Similarly, words like  what, which, whose, whom are used in place of nouns to describe a noun preceding (Placed before) them. These words are called Relative Pronouns. 

D. Interrogative Pronoun. : -A Pronoun that is used for asking a question is called an 'Interrogative Pronoun.'

For example : - What is you name?  Whom did you hand over the books?

Note the difference between the usage of 'which' and 'what' as an adjective and as a pronoun through the following examples:

Interrogative Pronouns Interrogative Adjectives
Which is your pencil? Which pencil is yours?
Whose is that umbrella ? Whose umbrella is that?
Which is your favourite subject? Which subject is your favourite?

Thus, when the interrogative words like which, whose and what are placed before a noun, they become interrogative adjectives.

Interrogative Pronoun is not followed by a Noun.
Interrogative Adjective is followed by a Noun.

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