Notes from ¿Qué tal? - Direct Objects and Personal a

In English and in Spanish, the direct object (el complemento directo) of a sentence is the first recipient of the action of the verb.

I see the car; but I don’t see the dog.
George is preparing dinner for the family

In Spanish, the word a immediately precedes the direct object of a sentence when the direct object refers to a specific person or persons. This a, called the a personal, has no equivalent in English.[1]

Vamos a visitar al profesor. – We’re going to visit the professor
but
Vamos a visitar el museo. – We’re going to visit the museum.

Necesitan a la camarera. – They need the waitress.
but
Necesitan la cuenta. – They need the bill.

The personal a is used before the interrogative words ¿quién? and ¿quiénes? when these words function as direct objects.

¿A quién llama? – Whom are you calling?
but
¿Quién llama? – Who is calling?

The personal a is used before alguien and nadie when these words function as direct objects.

¿Vas a invitar a alguien? – Are you going to invite someone?
¿A quién llamas? – Whom are you calling?
No llamo a nadie. –I’m not calling anyone.

Note: The verbs buscar (to look for), escuchar (to listen to), esperar (to wait for), and mirar (to look at) include the sense of the English prepositions for, to, and at. These verbs take direct objects in Spanish (not prepositional phrases, as in English).

Busco mi abrigo. – I’m looking for my overcoat.
Espero a mi hijo. – I’m waiting for my son.

Direct Object Pronouns
me – me
te – you (fam. sing.), him, it (masculine)
lo – you (form. sing.), him, it (masculine)
la – you (form. sing.), her, it (feminine)
nos – us
os – you (form. pl.), them (masculine, masculine + feminine)
los – you (form. pl.), them (masculine, masculine + feminine)
las – you (form. pl.), them (feminine)

Like direct object nouns, direct object pronouns (los pronombres del complemento directo) are the first recipient of the action of the verb. Direct object pronouns are placed before a conjugated verb and after the word no when it appears. Third person direct object pronouns are used only when the direct object noun has already been mentioned.

¿El libro? Diego no lo necesita. – The book? Diego doesn’t need it.
¿Donde estãn el libro y el periódico? Los necesito ahora. – V/here are the book and the newspaper? I need them now.
Ellos me ayudan. – They’re helping me.

The direct object pronouns y be attached to an infinitive.

Las tengo que leer. Tengo que leerlas. – I have to read them.

Note that the direct object pronoun lo can refer to actions, situations, or ideas in general. When used in this way, lo expresses English it or that.
Lo comprende muy bien. – He understands it (that) very well.
No lo creo. – I don’t believe it (that).
Lo sé. – I know (it)

[1] The personal a is not generally used with tener: Tengo cuatro hijos.
In Spain and in some other parts of the Spanish-speaking world, le is frequently used instead of lo for the direct object pronoun him.