An auxiliary verb is a verb that combines with a non-finite form (e.g., a participle or infinitive) of another verb to form a periphrastic construction. The auxiliary verb expresses such things as tense, mood, voice, and aspect, while the other verb provides the lexical meaning. For example, in "I had been walking," "had" is an auxiliary verb indicating the tense, "been" is an auxiliary verb indicating the aspect, and "walking" is a participle of a lexical verb identifying the action.