Most verbs are main verbs, i.e. content words that contribute meaning to the sentence they are in (go, run, eat, prefer, ascertain). In addition, there is a closed class sub-group of auxiliary verbs that merely assist in the technical construction of other verb forms. The auxiliary verbs in English are: to be, to do, and to have. The modal verbs (can, could, shall, should, may. might, must, will, and would) can also be considered auxiliaries as they modify the meaning of the main verb they accompany.
Dan is studying for a test at the moment.
[is, form of be, auxiliary verb of progressive verbs, here of the main verb studying]
French has been studied at this school for the past 10 years.
[has, form of have, auxiliary verb of perfect verbs, been, form of be, auxiliary verb of passive verbs, both are here of the main verb studied]
The weather forecaster will announce that it may rain tomorrow.
[ will, modal auxiliary verb of the main verb announce to form the “future simple tense”, may, modal auxiliary verb of the main verb rain, modifying it to denote the probability of the action]
The verb forms used as auxiliary verbs in English are:
1. to be
am, is, are was, were be, been, being
2. to do
do, does did
3. to have
have, has had having
4. The modal verbs
can, could shall, should must
may, might will, would
Keep these verbs in mind, as you encounter their uses in our English grammar articles.