Pronouns are used to refer to people and things that have already been mentioned. The main pronouns you need to be familiar with are:
a) subject pronouns (like I, you, she, etc.)
b) object pronouns (like him, them, me, you, us)
c) relative pronouns (like who, that, which)
d) possessive pronouns (like mine, yours)
e) reflexive pronouns (like myself and herself)
In order to successfully write with pronouns, you need to familiarize yourself with the different forms, the contexts in which they are used, and the order in which they appear.