The most common auxiliary verbs are "be," "do," and "have", and you may also use these verbs on their own. You use "Will" and "shall" to express future time.

In each of the following examples, a verb commonly used as an auxiliary verb appears as a simple predicate:

She is the chief engineer.
The tea cups are in the china cabinet.
Garth does this kind of thing frequently.
My roommates and I do the laundry every second week.
I can't complete my assignment because he still has my notes.
They have several kinds of gelato in the display case.

Other common auxiliaries are "can," "could," "may," "might," "must," "ought," "should," "will," and "would." A verb like these is called amodal auxiliary and expresses necessity, obligation, or possibility.

The highlighted word in each of the following sentences is a modal auxiliary:

Zora was pleased to learn that she could take several days off.
The small freckled girl told her neighbours that she would walk their dog for an appropriate fee.
Henry told Eliza that she ought to have the hole in the bucket fixed.
The principal told the assembled students that the school board might introduce a dress code next autumn.
According to the instructions, we must leave this goo in our hair for twenty minutes.

Several words may intervene between the auxiliary and the verb which goes with it, as in the following sentences:

They have not delivered the documents on time.
The treasure chest was never discovered.
The health department has recently decided that all high school students should be immunised against meningitis.
Will you walk the dog tonight?
The ballet corps was rapidly and gracefully pirouetting about the stage.

Auxiliary Verbs "Be", "Do", "Have"

An auxiliary verb helps the main (full) verb and is also called a "helping verb." With auxiliary verbs, you can write sentences in different tenses, moods, or voices. Auxiliary verbs are: be, do, have, will, shall, would, should, can, could, may, might, must, ought, etc. 
  • I think I should study harder to master English.
  • am having a cup of coffee.
  • You have been practicing hard.
  • It was written by a petitioner.
  • You may choose what you like.
The verb forms of be, do, and have can be used either as a main (full) verb or an auxiliary verb. The following examples show these verbs used as auxiliary verbs. 

1. "Be" as an auxiliary verb

a. Used in progressive sentences:

  • I am taking a bath.
  • She is preparing dinner for us.
  • They have been studying all night.
b. Used in passive sentences:

  • I was given a free meal.
  • He was seen by fans at the airport.
  • This song has been sung by all nations.
2. "Do" as an auxiliary verb

a. Used in negative sentences:

  • I do not know the truth.
  • She doesn’t agree with me.
  • They didn’t arrive here yet.
b. Used in questions:

  • Do you want to have another one?
  • Did he finish his homework?
  • Do we need to keep going straight?
3. "Have" as an auxiliary verb

a. Used in perfect sentences:

  • I have been following you for a mile.
  • We have done a lot so far.
  • She had been queen of the town.