Auxiliary verbs are helping verbs that combine with main verbs to form questions, negatives, tenses, aspects, voice, and modality. In English, the core auxiliaries are be, have, and do, and the modal auxiliaries include can, will, may, might, must, shall, should, would, and could. An auxiliary usually appears before the main verb and may determine word order in questions and negatives.

Be is an auxiliary used to form continuous tenses and the passive voice. With continuous tenses, be appears before a present participle, and with passive constructions, be appears before a past participle. The form of be changes for tense and subject agreement, not the main verb that follows.

Subject	Form
I	am
You	are
He she it	is
We	are
They	are
Past simple	was were
Perfect or future	been

Have is an auxiliary used to form perfect tenses, combining with a past participle. The present perfect uses have or has with a past participle, and the past perfect uses had with a past participle. The auxiliary have carries tense and subject agreement, while the participle stays unchanged.

Subject	Form
I	have
You	have
He she it	has
We	have
They	have
All subjects (past)	had

Do is an auxiliary used to form questions and negatives in the present simple and past simple, except when another auxiliary or a modal is present. In the present simple, do or does appears before the base form of the main verb, and in the past simple, did appears before the base form. The auxiliary do carries tense and subject agreement, not the main verb.

Subject	Form
I	do
You	do
He she it	does
We	do
They	do
All subjects (past)	did

Modal auxiliaries express possibility, necessity, permission, ability, or likelihood and are followed by the base form of the main verb. Modals do not change for subject or tense and do not take to before the main verb. In negatives and questions with modals, the modal stays first and the main verb stays in base form.

Word/Phrase	Definition
can	Can expresses ability or permission
could	Could expresses past ability or polite possibility
will	Will expresses future intent or prediction
would	Would expresses polite intent or hypothetical situations
may	May expresses permission or possibility
might	Might expresses possibility with less certainty
must	Must expresses necessity or strong conclusion
should	Should expresses advice or expectation
shall	Shall expresses formal future or offers in some varieties

Auxiliary constructions typically follow the pattern: subject plus auxiliary plus main verb form, with variations for tense and aspect. Multiple auxiliaries can appear in a sequence, such as have been, where each auxiliary selects a specific verb form. Negation usually inserts not after the first auxiliary, and contraction is a separate choice of form, not a change in auxiliary function.

Rule
In questions, the first auxiliary comes before the subject
In negatives, not follows the first auxiliary
With have been, have selects been and be selects the participle
With modals, the main verb stays in base form without to

Auxiliary verbs support main verbs by marking tense, aspect, voice, questions, and modality. Be, have, and do handle core tense and structure, while modals add meaning without changing form. The first auxiliary in a clause controls word order and negation.