Pronouns are substitutes for nouns. They are labor saving devices. We could say everything which we need to say without them, but at the expense of much repetition of longer words. A child often says "John wants Henry's ball" instead of "I want your ball." Constant remembrance of this simple fact, that a pronoun is only a substitute for a noun, is really about all that is needed to secure correct usage after the pronouns themselves have once become familiar. A construction which appears doubtful can often be decided by substituting nouns for pronouns and vice versa.
A very common error is the use of the plural possessive pronouns with the words any, every, each, somebody, everybody, and nobody, all of which are always singular.
We could accomplish this if every one would do their part.
is wrong. It should be
We could accomplish this if every one would do his part.
Another common mistake is the confusion of the nominative and objective cases in objective clauses where two pronouns or a noun and a pronoun occur.
All this was done for you and I.
is a very common but entirely inexcusable mistake. One would hardly think of saying
"All this was done for I."
I saw John and he leaving the shop.