grammar.prepositions.a-personal
The personal a: veo a María
A specific person (or pet) as direct object takes a: conozco a tu hermano, busco a mi perro. No a after tener or for unspecific people.
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Explanation
When the direct object of a verb is a specific, definite person, Spanish inserts the preposition a before it — the 'personal a'. So you see a person (veo a María) but see a thing (veo el coche): llamé a mi madre, conozco a tu hermano, vi a los vecinos. It has no English translation and must never be rendered as 'to'.
It also applies to personified pets (paseo a mi perro) and to the indefinite people-pronouns alguien, nadie, alguno, ninguno: no veo a nadie, ¿buscás a alguien? When quién asks about a person object, the a appears too: ¿a quién llamaste?
Skip the personal a in three cases: after tener meaning 'have' (tengo dos hermanos, not *tengo a dos hermanos), with non-specific or hypothetical people (busco un plomero — any plumber; compare busco a mi plomero — a specific one), and of course with things (veo el problema).
Examples
Do you know the owner?
Region: global
I'm looking for a (any) lawyer who speaks French.
Region: global
I saw your sister at the market.