Past Perfect

The past perfect tense corresponds to the English "had + past participle."  It indicates an event that happened prior to another event in the past.  It consists of the imperfect of haber and a past participle.  Sometimes the preterite of haber is used, but the imperfect is more common.

Carlos había vivido en México.  Carlos had lived in Mexico.
Habíamos aprendido el español.  We had learned Spanish.