When a person is the direct object of a verb. That person's name is always preceded by an a.

The a isn't used after tener.

The a is also before geographical names that aren't modified.

Whenever the prepositions a or de are followed by the article el, the two fuse together.

*pescado refers to fish when you are going to cook or eat it. Otherwise it is un pez.

When a pronoun is an object of a preposition, normally a subject pronoun should be used.

This isn’t the case for the first and second person singular forms- mí and tí.

In all other cases subject pronouns are used.

Con is special. In the first and second person singular forms, the preposition and pronoun combine and you get conmigo and contigo.