Pronouns can be used instead of nouns.

The main pronouns are:

I       you      he      she     it      we      they

When you are writing about a person, if a name is repeated too often sentences do not flow naturally. Instead of using the name every time, you can use a pronoun. Instead of a man’s name you would use he and instead of a woman’s name you would use she. If you are writing about more than one person, you use they.

John went into the house. John He picked up his newspaper from the mat.

Mary read the letter again. Mary She threw it into the bin.

John and Mary were angry. John and Mary They decided to go and talk to the council.

Pronouns are also useful when you are writing about a thing. Instead of repeating the same word, you can use it or they.

The bus was red. The bus It had fifty-seven seats.

I saw seven boats. The seven boats They were sailing close to the port.

A pronoun can also stands in for a noun phrase.

For example:

Joe asked Mrs Redding [noun] to repeat what she had said [noun phrase].

Using pronouns, the sentence becomes:

Joe asked her to repeat it.

I and we are first person.

You is second person.

All other subjects are third person (both pronouns - such as he/she/it - and nouns)

First person                 I am                                       We are

Second person           You are                      

Third person                He/she/it is                           They are     

                                    The insurance company is   Passengers/trespassers are 

The third person has traditionally been used in formal documents, although some documents are now written in the second person to sound more friendly and personal. Look at these examples:

Job applicants are asked to provide references.

We ask you to provide references when you come for interview.

The first sentence is more formal. It uses the passive voice and the third person.
The second sentence is less formal. It uses the active voice and the first and second person. It speaks more directly and personally to the reader.

Using pronouns clearly

I or me

Watch out for how you use the pronouns I and me, especially in phrases pairing up I or me with another person.  For instance:

my wife and I   

 the children and me.

Use I if referring to the subject of a sentence but me if referring to the object. 

To test whether I or me is right, try leaving the extra person out of the sentence. 

It is right to say:                   My friend and I went to the cinema.

                                                     (I [subject] went to the cinema.)

But, change I to me when used as an object:

                                       My husband took my friend and me to the cinema.

                                     (My husband  [subject] took me [ object] to the cinema.)