The personal “a”

Before a direct object that is a specific person (or a pet), Spanish adds a:

• Veo a María. — I see María.
• Conozco al profesor. — I know the teacher.
• No conozco a nadie. ¿A quién llamas? — I don't know anyone. Who are you calling?

No “a” before a thing (Veo la casa), after tener (Tengo dos hermanos) or before any person at all: Busco un médico (any doctor) / Busco a un médico (a particular doctor).

Why? Word order is free in Spanish and the subject often comes after the verb. The “a” shows who receives the action: Llama a Juan. — He / She calls Juan. / Llama Juan. — Juan is calling.