30. Impersonal "a"

When the object of a verb (except tener) is a definite person, it is preceded by a. It isn't used if a number precedes the object though. The pronouns alguien (somebody), alguno (someone), nadie (nobody), and ninguno (no one) require a as well, when used as the direct object. It is also used before geographical names that are not already preceded by an article.

Veo a Juan.
 I see John.

Veo a alguien.
 I see somebody.

Describe a Chile.
Describe Chile.