What is the personal a in Spanish?

The personal a is a small word Spanish puts before a direct object when that object is a specific person (or a pet you treat like one). Veo a María means "I see María". It doesn't translate to anything in English, and it's not the same a as "to". You simply need it with people.

The basic rule

When the thing receiving the action is a specific person, put a before it.

• Conozco a tu hermano. (I know your brother.)
• Llamo a mi madre todos los días. (I call my mother every day.)
• Busco a mi amigo. (I'm looking for my friend.)
• Quiero mucho a mi perro. (I love my dog a lot.)

When the object is a thing, no a:

• Veo la casa. (I see the house.)
• Busco las llaves. (I'm looking for the keys.)

Compare Veo a Pedro with Veo el coche. The first needs a, the second doesn't.

Combining with el

If the person is introduced with el, a and el merge into al: Veo al profesor (I see the teacher). Conozco al director (I know the director). With la, las and los, they stay separate: Veo a la profesora.

Which things count as people?

Here's where it gets interesting.

• Pets and animals you know: Cuido a mi gato. (I look after my cat.)
• Specific groups of people: Escucho a los niños. (I listen to the children.)
• Personified things or countries (sometimes): Extraño a mi ciudad. (I miss my city.) This is rarer and a bit stylistic.
• Questions with "who": ¿A quién ves? (Who do you see?)

When you don't use it

• Non-specific people: Necesito un médico. (I need a doctor, any doctor.) But: Necesito al médico que me atendió ayer (I need the doctor who treated me yesterday).
• Unknown or any person: Busco un profesor de inglés. (I'm looking for an English teacher.) vs Busco a mi profesor de inglés. (I'm looking for my English teacher.)
• After tener in most meanings: Tengo dos hermanos. (I have two brothers.) No a.
• After hay: Hay tres personas. (There are three people.)

The pattern: the more specific and known the person is, the more likely you need a.

Is it just a rule for people?

Mostly, yes, but there's a second job. The same a appears before indirect objects (Le doy el libro a Ana), and that is the ordinary "to" meaning. Sometimes a sentence has both: Presento a Ana a mi madre (I introduce Ana to my mother). The first a is the personal one, the second is "to".

Common mistakes

• Dropping it: Veo mi hermano should be Veo a mi hermano.
• Adding it before things: Compro a un libro is wrong. It's Compro un libro.
• Overthinking it with tener: skip it. Tengo un amigo en México (I have a friend in Mexico).

Why it matters

Mostly clarity. In Spanish, word order is flexible (see Spanish word order), so the a tells you who's doing the action and who's receiving it: A María la ve Pedro clearly means that Pedro sees María, not the other way around.

My advice: don't fuss over it. Get it right with the common verbs (ver, conocer, llamar, buscar, escuchar) and it'll become second nature fast.

Hearing it in context beats rules here. In Langtok, the app I built, you can tap any word in a short Spanish story to translate it, so you'll notice the a before people as it goes by.