Subject pronouns tell who is doing the action. They replace nouns or names in a sentence. Subject pronouns are often used for ephasis or clarification:
Gregorio escucha música. El escucha música.
A prepositional pronoun is a special form of a personal pronoun that is used as the object of a preposition (and with a preposition.) English does not have distinct prepositional forms of pronouns. Spanish uses the same set of object pronouns after verbs and prepositions as well except for I and you forms:
EXCEPTIONS to the use of these pronouns are:
1. Preposition de is never used for possession with the 1st or 2nd singular person. In these cases you use the possessive adjective:
Ella es mi madre. NO *La madre de mí.
¿Es tu libro? NO *El libro de ti.
2. The preposition entre uses the personal pronouns instead of the prepositional with the 1st and 2nd singular person.
Solamente entre tú y yo, creo que es pura mentira. NO *ti y mí
3. Preposition con has the special form for 1st and 2nd singular person: conmigo and contigo
— ¿Quieres estudiar conmigo?
— Sí, me gusta estudiar contigo.