Personal pronouns are used to talk about yourself, the people you are talking to, the people you are referring to.
Subject pronouns (I, you, he, she, it, we, they) refer to the subject of a clause, while object pronouns (me, you, him, her, it, us, them) are used as objects after verbs and prepositions.
According to traditional grammar, we should use subject pronouns after linking verb to be. Now object pronouns are commonly used in these structures:
We usually use object pronouns after except and but (= except). Subject pronouns in these structures are old-fashioned and may sound awkward.