An auxiliary verb is a verb that establishes the mood, tense, voice, or aspect of a different verb in a verb phrase. Auxiliary verbs are, do, and have, and modals like can, might, and will, can be juxtaposed with main and lexical verbs.

Auxiliaries are also referred to as helping verbs because they help to complete the meaning of the main verbs. Distinct from main verbs, auxiliary verbs can’t be the verb in a sentence except in elliptical expressions where the main verb is assumed as if it were present.

Auxiliary verbs precede main verbs within a verb phrase, such as in the sentence “You will help me.” However, in interrogative sentences, the auxiliary is in front of the subject, as in “Will you help me?”

The standard for English grammar, set by “The Cambridge Grammar of the English Language” and other similar university press releases, defines auxiliary verbs of English as “can, may, will, shall, must, ought, need, dare” as modals and “be, have, do, and use” as non-modals.