Double-click on any word and see its definition from Cambridge Dictionaries Online.

For a succinct description of the use of auxiliary verbs, we turn to Swan (Practical English Usage, Oxford University Press, Second Edition, 1995):

"The need for auxiliary verbs

In English sentences, a lot of important meanings are expressed by the form of the verb phrase - for example questioning, negation, time, completion, continuation, repetition, willingness, possibility, obligation.

But English verbs do not have many different forms: the maximum (except for "be") is five (e.g. see, sees, seeing, saw, seen). So to express all these meanings, a number of 'auxiliary' (or 'helping') verbs are added to other verbs. There are two groups:

1) be, do and have

Be is added to other verbs to make progressive or passive forms.

Is it raining?

She was imprisoned for three years.

Do is used to make questions, negatives and emphatic forms of non-auxiliary verbs.

Do you smoke?

It didn't matter.

Do come in.

Have is used to make perfect forms.

What have you done?

I realised that I hadn't turned the lights off.

2) modal auxiliary verbs

The verbs will, shall, would, should, can, could, may, might, must and ought are usually called 'modal auxiliary verbs'. They are used with other verbs to add various meanings, mostly to do with degrees of certainty or obligation."
p. 83

For a brief description of modal verbs, see our Grammar definitions section. For a more detailed look at the modal verb would, see our grammar archive.

For more information about auxiliary verbs, see the following web sites:

General

http://webster.commnet.edu/grammar/auxiliary.htm

Quiz

http://perso.wanadoo.es/autoenglish/quest-quiz.htm