A pronoun takes the place of a noun.

For example,

The new phone is designed for business use, but it can take hi-resolution pictures as well 

The pronoun ‘it’ replaces the noun ‘phone’.

Agreement between noun and pronoun

The pronoun must agree in number with the noun that it replaces.

For example,

The committee was asked to submit its final report by Thursday

In the above example, the noun 'committee', being a collective noun, is singular.

Therefore, it takes the singular pronoun ‘its’

For example,

The committee members felt they needed some more time to deliberate on the issue

In the above example, the pronoun refers to the (committee) members (plural) and not to the committee (singular). 

Therefore, the pronoun takes the plural form ‘they’

Correct use of Pronoun Case

Pronouns given below can be used in three forms:

Subject Form

 – where the pronoun is the subject of the verb in the sentence

For example,

Correct: We are going for a walk

Incorrect: Us are going for a walk

In the above example, the pronoun ‘we’ is the subject of the verb ‘to go’.

‘We’ is the subject form. The object form of the same pronoun is ‘us’.

Therefore, it is incorrect to say ‘Us are going for a walk’

Object Form

– where the pronoun is the object of the verb in the sentence

For example,

Correct: The girls bought us some chocolates

Incorrect: The girls bought we some chocolates

In the above example, ‘girls’ are the subject of the verb ‘to buy’.

We are the ones for whom chocolates are being bought. But we cannot use ‘we’ in the object form. We must use ‘us’

Possessive Form

– where the pronoun indicates ‘belongs to’

For example,

Correct: Where are our chocolates?

Incorrect: Where are us chocolates?

In the above example, we are referring to ‘chocolates belonging to us’.

Therefore we use the possessive form of the pronoun i.e. ‘our’