Quick answer
Veo a María · Veo la casa
When the direct object is a person, Spanish puts a in front of it. English has nothing like it, so it gets skipped.
People get an a, things do not
Compare the pairs. The verb and the structure are identical; only the object changes.
Remember that a + el contracts to al: Llamo al médico.
What else counts as a person
| Object | a? |
|---|---|
| a named person | yes |
| a pet with a name | yes |
| alguien, nadie, quién | yes |
| an unspecified person (un médico, in the sense of any) | usually no |
| a thing, a place, an idea | no |
The verb tener is the exception
tener normally drops the personal a, even with people.
It comes back when tener means holding or keeping someone somewhere: Tiene a su madre en el hospital.
Memory trick
Ask what the verb is acting on. If it is a specific human being, an a goes in front — and it disappears again after tener.
Typical mistakes
Veo mi madre todos los días.
Veo a mi madre todos los días.
A specific person as direct object needs the a. English gives you no reminder at all.
Llamo el médico.
Llamo al médico.
The a is required and merges with el into al.
Tengo a dos hijos.
Tengo dos hijos.
Tener drops the personal a in the plain sense of having family members.
Check yourself
¿Conoces ___ mi hermana?