Two of the personal pronouns, you and it, have only one form that is used as either subject or object:
See that goat? It bit me. (It is the subject of the verb bit.)
I hear a bee. Do you see it? (It is the object of the verb see.)
There’s a poisonous spider. Step on it! (It is the object of the preposition on.)
You agree with Charlie on everything. (You is the subject of the verb agree.)
That car just missed you. (You is the object of the verb missed.)
This information must remain between you and me. (You is the object of the preposition between.)
Five of the personal pronouns have two forms each: a subject form (I, he, she, we, they) and an object form (me, him, her, us, them).
The most common errors occur when subject and object forms are reversed.
Two additional errors that seem to be increasing are (a) replacing a personal pronoun with a pronoun ending in -self and (b) using a personal pronoun in a context that calls for a possessive adjective.