Auxiliary verbs are irregular verbs that provide information about other types of verbs. The main auxiliary verbs in English are to have, to be, and their conjugated forms, while others include can, could, did, do, may, should, and would.
To be and its inflected forms (am, are, be, been, being, was, and were) have several functions, but they’re most commonly used as auxiliary verbs.
When to be is placed before a present-participle verb, it denotes continuing action—for example:
I am talking on the phone.
We were making headway.
When to be is placed before a past-participle verb, it makes the verb passive—for example:
The ball is passed back to the goalie.
The stadium was built in 1906.
Do, does, did
The auxiliary verbs do, does, and did are used to give emphasis. When used for this purpose, they’re always followed by present-tense verb stems—for example:
I do like chocolate truffles.
We did do our homework.
(The do in the second example shows how do can function as a principal verb as well as an auxiliary verb.)
Have, has, had
When used before past-participle verbs, the auxiliary verbs have, has, and had form the perfect tense. Have and has create the present-perfect tense:
I have run.
She has been to China.
Had is used to create the past-perfect tense:
I had been running.
She had been in China.
Will have is used to form the future-perfect tense:
I will have gone for a run.
She will have come back from China.