In Spanish, the direct object — the object that receives the action of the verb — is preceded by an a if the direct object is a specified person.
I'm looking at the television. | Miro la televisión. |
I'm looking at you. | Miro a usted. |
Pets or other animals we care about are also sometimes considered persons in this sense, but not all animals are.
I'm looking at the fox on the television. | Miro el zorro en la televisión. |
I'm looking at my dog. | Miro a mi perro. |
Persons are only preceded by a when they are specified. Below, in the second sentence you are looking at a specific doctor, whereas in the first sentence any doctor at all is fine, you don't need a specific doctor.
I need a doctor. | Necesito médico. |
I'm looking at a doctor. | Miro a un médico. |
Personal pronouns are preceded by a when they are direct objects.
I'm looking for someone. | Busco a alguien. |
I'm not looking at anyone. | No miro a nadie. |
Whom are you looking for? | ¿Buscas a quién? |
When using the verbs tener and hay, the direct object is not preceded by a.
I have a girlfriend. | Tengo novia. |
There is a boy in the street. | Hay un niño en la calle. |
Learning this might seem complicated now, but in time your intuition will tell you when to use and when not to use a.