Spanish Grammar in English

Besides subject pronouns there are prepositional, or tonic, pronouns, which are used only with prepositions, and object pronouns, which are used with verbs:

The prepositional pronouns coincide with the subject pronouns except for three forms: mí, ti and sí. The latter is normally used with the word mismo (himself/herself):

Esta chica sólo piensa en sí misma = This girl only thinks of herself

OI pronouns answer the question "to whom", while OD pronouns correspond to "what / who":

to whom: le, les

who: lo/la, los/las

Object pronouns must duplicate any object which stands before the principal verb, compare:

Quiero mucho a Marta = I love Marta a lot
A Marta la quiero mucho = I love Marta a lot

Voy a regalar un libro a Marta = I am going to give Marta a book
A Marte le voy a regalar un libro = I am going to give Marta a book

OI pronouns have no gender. Third person OD pronouns yes:

A María le escribo mañana / A Pedro le escribo mañana = I am writing to Maria tomorrow / I am writing to Pedro tomorrow

A María la vi ayer en la tienda = I saw Maria yesterday in the store

A Pedro lo vi ayer en la tienda = I saw Pedro yesterday in the store

Forms lo, los can be changed to le, les when stand for people:

A Pedro le vi ayer en la tienda

Prepositional pronouns are never used alone with verb forms:

INCORRECTO: Escribo *a ti

CORRECTO: Te escribo = I`ll write to you

However, they can be used together with the object pronouns to clarify things:

Te escribo a ti (y no a Juan)