I. The preposition "a" is used:
1. With negative words:
a nadie, a ningún, a ninguna, a ningunos, a ningunas
2. With certain verbs:
ir a, aprender a, comenzar a, conocer a (when talking about a person), empezar a, enseñar a, ayudar a
3. Before the subject with pronominal verbs such as gustar, interesar, encantar:
A Miguel le gusta jugar
A mí me encanta leer
A ti te interesa la clase
II. The preposition "a" is always contracted when followed by the indefinite article el with the only exception of being part of a proper name:
a + el = al
Vamos a El Salvador