Auxiliary Verbs (from the Latin auxilio = to help; they are also known as helping verbs) are verbs‏‎ used to change the tense, form‏‎ mood and voice of other verbs.

In other words, we add an auxiliary to a verb to turn it into a question, to put it into the past, to make it negative and so on.

The 3 English Auxiliaries Verbs

There are three auxiliary verbs: be, have and do.

They are used before the main verb to make different verb forms, negatives‏‎ and questions‏‎. One or more auxiliary verbs can be used together.

Different verb forms are made thus:

{auxiliary} + [auxiliary] + {main verb}

She has + been + waiting for three hours.
We had + talked for over an hour.

The only verb forms where we do not use an auxiliary verb are the present simple‏‎ and the past simple‏‎.

Negative statements are made thus:

{auxiliary} + {not} + [auxiliary] + {main verb}

They have + not + been + waiting long.
He is + not + living here anymore.

To make questions we use this pattern:

{auxiliary} + {subject} + [auxiliary] + {verb}

Has + he + been + watching television?
Do + you + want a cup of tea?

Note that we can also use be, have and do as lexical verbs, that is, using them as a simple verb and not using them as auxiliary verbs:

I am English.
They have thirteen cats in their house.
She did it!

Useful Links

The ICAL Grammar Foundation Course – a course in English grammar.

Verbs in English Grammar – an overview of verbs in English