Questions about Spanish prepositions:
Q: Why is it Voy a México and not Voy al México?
Spanish generally does not use the definite article el or la in front of countries, just bare preposition a. Some exceptions of places that require an article include El Salvador, el Reino Unido, la República Checa, and la Repúlica Dominicana. Some other countries optionally use an article, so you may see it but it is not required.
Q: Why is it Nosotros buscamos una casa grande and NOT Nosotros buscamos por una casa grande?
Unlike English, the verb buscar ("look for"/"seek") is not followed by a preposition in Spanish. The noun comes directly after it.
Q: Why is it Yo visité a mis abuelos/Yo visité el zoológico and NOT Yo visité mis abuelos/Yo visité al zoológico?
Before objects that refer to people/animals, you must use the personal a. However, do not use the personal a before other objects, unless the verb requires a (see table above).
Q: Why is it él juega al fútbol but NOT él juega al piano?
When you mean "play an instrument", use the verb tocar without a preposition, e.g.: él toca el piano; when you mean "play a game/sport", use jugar a with the preposition a.
Q: Why is it Voy a ayudar a él/Voy a ayudar al niño and NOT Voy a ayudar al/Voy a ayudar a el niño?
The preposition a contracts with the article el (no accent) to form al, but it does NOT contract with the personal pronoun él (accent) and combinations of these must be written as separate words a él.
Q: Why is it Compré el regalo para mi madre and NOT Compré el regalo por mi madre if I bought the gift to give to my mother?
Although both sentences are grammatical, they mean different things. Use para to indicate a recipient, as in this example. If you use por in this example, it means you bought the gift as a favour to your mother so she could give it to someone else.