What parts of speech replace the words that serve as subjects and can serve as subject themselves?

Pronouns replace a noun, a pronoun, or nounphrase. Examples:

Mickey rode the bus to Liberty Street where he got off. (the pronoun 'he' replaces the noun 'Mickey')

He and she both accepted. Theywill be joining us. (the pronoun 'they' takes the place of the pronouns 'he and she')

The man next door has a garden and he gave me these flowers. (the pronoun 'he' replaces the noun phrase 'the man next door')