The personal a.

When the direct object of a verb is a specific person or specific group of persons, it is preceded by the personal a; if the noun is personal but indefinite, the a is not used.

Conozco a María.		I know María.		[María is a definite person, and the object of the verb conozco.]
Busco a un abogado; se llama José Jiménez.		I'm looking for a lawyer; his name is José Jiménez.		[Since it's a definite person and the object of the verb buscar, the personal a is used.]
Busco un abogado; tengo problemas con una multa.		I'm looking for a lawyer; I've got problems with a traffic ticket.		[I need a lawyer and I don't know who I'll find; the personal a is not used since a definite person is not involved.]

The personal a is used with indefinite and negative pronouns —for example alguien (someone), nadie (no one, nobody), quienquiera (anyone [at all], whosoever)— when they are used as direct objects and referring to persons.

No veo a nadie aquí.		I don't see anyone here.
Conozco a alguien que puede ayudarnos.		I know someone who can help us.

The personal a is not used after the verb tener, unless it is used to mean “enrolled in” or “located in”.

Tengo dos hijas.		I have two daughters.
Tengo a dos hijas en esa escuela.		I have two daughters in that school.

Some speakers use the personal before proper place names when they are direct objects, and some will use it with the names of animals --especially pets-- almost humanizing them:

Juan siempre dice “¡Conozco bien a Madrid!”.		Juan always says, “I know Madrid well!”
¡Ay, has lastimado a Quiqui, un  perrito que no le ha hecho daño a nadie!		Oh, you've hurt Quiqui, a doggy who's never harmed anybody!

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