An auxiliary verb modifies the main verb to give more information about the main verb.
In Italian, compound tenses are formed with an auxiliary verb (either essere "to be" or avere "to have"). The distinction between the two auxiliary verbs is important for the correct formation of the compound tenses and is essential to the agreement of the past participle.
Transitive verbs use avere as their auxiliary verb. For intransitive verbs, there are several rules, depending on a verb. Reflexive verbs use essere (typically non-agentive verbs of motion and change of state. Some verbs may use both auxiliary verbs with an inherent change in meaning.
