Personal a

The personal a marks a specific human direct object: veo a María, but veo la casa. The exception that catches over-correctors is tener, which drops the a for plain possession. Tengo dos hermanos, never tengo a dos hermanos. The a comes back only when tener means keeping someone rather than having them: tengo a mi madre en casa.

When the person or thing a verb acts on is a specific person, Spanish puts a in front of it, and English translates nothing: veo a María, but veo la casa. Loved pets, alguien and nadie take it too: no veo a nadie. A + el contracts as always: conozco al profesor.