What Are Auxiliary Verbs?

In English grammar, an auxiliary verb is a verb that determines the mood, tense, voice, or aspect of another verb in a verb phrase. Auxiliary verbs include be, do, and have along with modals such as can, might, and will and can be contrasted with main verbs and lexical verbs.

Auxiliaries are also called helping verbs because they help to complete the meaning of main verbs. Unlike main verbs, auxiliary verbs can't be the only verb in a sentence except in elliptical expressions where the main verb is understood as if it were present.

Auxiliary verbs always precede main verbs within a verb phrase such as in the sentence "You will help me." However, in interrogative sentences, the auxiliary appears 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 (having no infinitive form) and "be, have, do, and use" as non-modals (which do have infinitives). 

To Be or Not to Be Helping Verbs

Since some of these words are also "to be" verbs, which can operate as main verbs, it's important to know the distinctions between the two. According to the "American Heritage Guide to Contemporary Usage and Style," there are four ways in which auxiliary verbs differ from main verbs.

First, auxiliary verbs do not take word endings to form participles or agree with their subject, and thus it is correct to say "I may go" but incorrect to say "I mays go." Secondly, helping verbs come before negative clauses and don't use the word "do" to form them. The main verb must use "do" to form the negative and follows not like in the sentence "We do not dance." 

Helping verbs also always come before the subject in a question, whereas main verbs use "do" and follow the subject to form questions. Therefore, the word "can" in the question "Can I have another apple?" is an auxiliary verb while "do" in "Do you want to go to the movies?" acts as the main verb. 

The final differentiation between the two forms of verbs is that auxiliary words take the infinitive without also needing the word "to," like in the sentence "I will call you tomorrow." On the other hand, main verbs that take an infinitive always have to use the word "to," such as "I promise to call you tomorrow."

A Limit to Helping

English grammar rules dictate that an active sentence may contain a maximum of three auxiliaries, while a passive sentence may include four, wherein the first is finite and the rest nonfinite words. 

Barry J. Blake breaks down the famous Marlon Brando quote from "On the Waterfront," where he says "I could've been a contender," by observing that in the example "we have a modal followed by the past participle of the verb 'to be.'"

Any more than three auxiliaries and the sentence becomes too convoluted to decipher. And, consequently, the helping word no longer helps clarify the main verb it's meant to modify.