The one question: is the object a person you have in mind?
Look at who or what receives the verb. If it is a person — or a pet, or a word that stands for people — and you mean someone in particular, put a in front of it: veo a mi madre, busco a Juan, está esperando a unos amigos, ¿a quién llamaste?, no conozco a nadie. If it is a thing, a place or an idea, nothing: veo la catedral, puso el libro en la mesa, conozco Madrid. The a is not the preposition to; it marks the object as a person, and English has no word for it, which is why learners drop it for years.
The academies' Diccionario panhispánico de dudas lists where it is required: names of people and pets (vi a Pedro, dejé a Pluto); people whose identity is concrete (vi a los hijos del vecino); a family or crowd you have in mind (visitar a su familia, dispersaron a la multitud); the stressed pronouns (me vio a mí); alguien, nadie and todos (llévate a alguien, no conozco a nadie, ¿invitaste a todos?); quién as an object (¿a quién buscas?); and the relatives quien and el que when they are the object (el hombre a quien golpearon — though plain que never takes it: las personas que amamos). The chart below puts the common cases side by side.
Every case, in one chart
The last four rows are the ones English speakers get wrong by reasoning instead of by ear: people are involved, so where is the a? The next three sections are about exactly those — the verbs that refuse it, the any-person reading that drops it, and places and things.
| English | Spanish |
|---|---|
| I see my mother | veo a mi madre |
| I'm looking for Juan | busco a Juan |
| who did you call? | ¿a quién llamaste? |
| I don't know anyone here | no conozco a nadie aquí |
| I took my dog to the vet | llevé a mi perro al veterinario |
| I admire the doctors who work nights | admiro a los médicos que trabajan de noche |
| I see the cathedral | veo la catedral |
| I have two brothers | tengo dos hermanos |
| there are a lot of people | hay mucha gente |
| I'm looking for a waiter (any waiter) | busco un camarero |
| I got to know Barcelona | conocí Barcelona |
When the a is optional, and changes the meaning
With an ordinary noun for a person, the academies describe a double use: the a appears when you mean someone specific and disappears when you mean anyone of that kind. Busco un camarero is any waiter, a member of the class; busco a un camarero is a particular waiter you already know. The verb that follows gives it away — busco un camarero que sepa inglés takes the subjunctive, busco a un camarero que sabe inglés the indicative. The same split runs through verbs of looking for, needing, preferring and wanting (quieren al mejor defensa or quieren el mejor defensa), verbs of choosing and hiring (han contratado (a) otro colaborador, hay que nombrar (a) un secretario), llevar and traer (llevaré (a) unos amigos a la fiesta), and ver and conocer (vi (a) algunos niños en el jardín, ¿conoces (a) un buen abogado?). Other verbs of perception — mirar, observar, oír — keep the a: estaba mirando a una señora.
One group takes the a even when the person is general: verbs of harm or benefit — acompañar, admirar, amar, engañar, golpear, odiar, saludar and the like. Acompañó a una anciana hasta su casa; admiro a los políticos que saben defender sus opiniones; engañar a un jefe es imposible are the academies' examples, and admiro a los médicos in the quiz is the same rule.
The verbs that break the rule: haber, tener and the bare plural
Haber never takes the personal a, however human the object: hay alguien en la puerta, solo había dos estudiantes, no hay quien lo entienda. Tener with people you simply have goes without it — tienen dos hijos, tiene una tía actriz — and takes it only when something is said about the person's state or place: tengo a mi madre enferma, tiene a su jefa muy contenta, aquí tienes al novio de tu hija. And a person-noun with no article at all, in the plural or as a type, goes bare: he encontrado camareros para mi nuevo bar, llevaré amigos a la fiesta, ¿buscas pareja?, necesito acompañante para la boda — unless two such nouns are joined, which forces the a back (el público silbó a árbitros y jugadores), or the noun carries a modifier that singles people out (conozco (a) personas influyentes). Verbs of causing stand apart too: el accidente causó varios heridos answers what, not whom, and takes no a.
Pets, places and things
Animals take the a in proportion to how close they are to the speaker, in the academies' words: suelta al caballo for a horse you care about, suelta el caballo for one you do not — so pets almost always get it (llevé a mi perro al veterinario) and wild or anonymous animals almost never do. Countries, cities and continents as objects go without it in today's Spanish — no conozco Francia, he visitado Nápoles, recorrió América en bicicleta — the older conocer a Francia having practically disappeared, unless the name stands for its people (es capaz de engañar a media Italia). Other place names never take it: cruzó el Tajo, escaló el Himalaya.
Things take no a — regaló un automóvil a su padre — with two narrow exceptions the academies record. When both the subject and the object are things and the sentence would otherwise read two ways, the a marks the object: el espíritu que alienta a este pontificado, though the academies prefer reordering the sentence instead. And verbs of sequence, seguir and suceder, keep it: la calma sigue a la tempestad. Personification can bring it in too — espera a la muerte con serenidad — but that is a writer's choice, not a rule you need.
Why do English speakers drop it?
Because nothing in English corresponds to it. I see my mother and I see the cathedral have the same shape, so a learner translating word for word produces veo mi madre, which every Spanish speaker hears as a foreign accent in grammar. It is taught in the first weeks and still goes missing at B1 and B2, because it is not a rule you can reason your way to mid-sentence; it has to become a reflex, and the only way it becomes one is saying the sentences with it until the a comes out on its own. That is what the quiz and the speaking practice are for.
There is no first-party error rate on this page yet: our mistake tagger counts ser vs estar, por vs para, the tenses and the object pronouns — the pronouns that stand in for these very objects — not the a before them. When it does, the number will sit here. Until then the quiz is the measure.
What comes after the personal a?
Lo, la, le are the pronouns that replace a direct object once you have named it — veo a mi madre becomes la veo — and the le/lo choice turns on the same person-or-thing line. Pedir vs preguntar is where the a is the ordinary preposition to, a different animal (le pido un favor a Juan). The free Spanish level test says where all of this sits on the CEFR scale in five minutes.