Objective case, but not the same as object pronouns
Spanish prepositional pronouns are used the same way as in English: as the pronoun object of a preposition. In English, we use the objective pronoun form (me, her, him, us, them) both as the direct or indirect object and the object of a preposition. Spanish prepositional pronouns do not behave the same way.
Here’s a list of Spanish prepositional pronouns:
mí – me, myself
ti – you, yourself (familiar)
Ud – you, yourself
él – him, it
ella – her, it
sí – himself, herself, itself
nosotras/as – us, ourselves
vosotros/as – you, yourselves (Spain)
Uds – you, yourselves
ellos – them
ellas – them
sí – themselves
The good news about prepositional pronouns…
Notice that except for mí, ti, and sí, the Spanish prepositional pronouns are the same as the subject pronouns. Also, take a look at the following examples:
A mí, no es importa. (It is not important to me.)
Para ella. hablar español es difícil. (For her, speaking Spanish is difficult.)
José se lo regala a él. (Jose gave it to him (i.e., someone else).
*José se lo regala a sí mismo. (José gave it to himself.)
* Special rule for sí: When the third person subject (José in the above example) refers to himself, herself, or itself, Spanish adds the adjective mismo/a to clarify the object.
The prepositional pronoun “con- game”
When the prepositional pronouns mí, ti, and sí are used with the preposition con (with), they become conmigo, contigo, and consigo.
¿Quieres ir conmigo al cine? (Do you want to go to the movies with me?)
Pedro siempre lleva su computador portátil consigo. (Pedro always brings his laptop with him.)
Clearing up the confusion of “le” or “les”
Spanish frequently uses the prepositional pronouns when a sentence contains an indirect object and there could be some confusion as to the meaning or intent. Note the following examples:
Les di el libro a Uds. (I gave you the book.)
Since les could refer to Uds, ellas, or ellos, we need to clarify with the prepositional phrase "a Uds."
Exceptions, always exceptions!
Two exceptions: The subject pronouns yo and tú are used with some prepositions (instead of mí and ti.) Those prepositions are entre (between), excepto/salvo (except), incluso (including) and según (according to). A few examples:
La diferencia entre yo y tú no es grande. (The difference between you and me is not great.)
Muchas personas incluso yo van a la iglesia todos los domingos. (Many people including me go to church every Sunday.)
Según ti, no es importa. (According to you, it is not important.)
Read more about Spanish object pronouns at:
About.com: Spanish Language – Prepositional Pronouns