A pronoun is a word that takes the place of a noun in a sentence. Common pronouns are he, she, it, their, they and so on. Pronouns may seem simple to use in your writing, but there are some very specific rules regarding their use which eliminate confusion.
1. Pronouns refer back to the last noun of the same number and gender. (Remember that number refers to singular/plural and gender refers to male/female.) If your pronoun does not refer back to that last noun (which is called the antecedent), then it is incorrectly used.
2. A pronoun must refer to a noun antecedent in the same paragraph. It follows, then, that you should avoid using a pronoun as the first word of any paragraph.