Pronouns are used to refer to people and things that have already been mentioned. The main types you should be familiar with are:
a) subject pronouns (e.g.: eu, tu, ele, ela, nós, vós, eles, elas)
b) direct object pronouns (e.g.: o, a, os, as, me, te, nos, vos)
c) indirect object pronouns (e.g.: lhe, lhes, me, te, nos, vos)
d) relative pronouns (e.g.: quem, que, o que, etc.)
e) possessive pronouns (e.g.: o meu, a tua, os seus, etc.)
To use pronouns correctly, you need to understand:
See the sections on each type of pronoun for detailed explanations.