Definition: A verb that is used with a main verb that follows to help indicate its tense or some other way in which the verb is understood. As such, the auxiliary verb often doesn't have any meaning in itself, only in how it affects the main verb. The auxiliary verb and the main verb together form what is known as a compound verb.
Auxiliary verbs are used much more often in English than they are in Spanish, since Spanish is able to use conjugation to indicate tenses that sometimes are expressed in English with auxiliary verbs. For example, the future tense in English uses the auxiliary verb "will" as in "I will study." But Spanish needs no auxiliary verb in this case, as the future is expressed through a verb ending: estudiaré. English also uses the auxiliary verb "do" to form many questions, as in "Do you study?" Such an auxiliary isn't needed in Spanish: ¿Estudias?
Two very common English auxiliaries have equivalents in Spanish: In English, forms of "to have" are combined with the past participle to form the perfect tenses; in Spanish, haber is used. In English, forms of "to be" are combined with the present participle to form the progressive (or continuous) tenses; in Spanish, forms of estar are used with the gerund.
Also known as: Helping verb. The Spanish term is verbo auxiliar.
Examples: Auxiliary verbs are in boldface; note that sometimes an auxiliary is used in one language but not the other. I have bought the medicine. (He comprado los medicamentos.) We are celebrating. (Estamos celebrando.) He does not work. (No trabaja.) I can swim. (Puedo nadar.) I know how to swim. (Sé nadar.) I usually drive fast. (Suelo manejar rápido.)