Main verbs often need helper verbs to complete their meaning, and these helpers are called auxiliary verbs.
For instance, you’re thinking of going out to dinner, so you say to your roommate: "I may go out to dinner tonight and try that new restaurant."
If go is the main verb, what is the auxiliary? Of course, it’s may.
Auxiliary verbs are often arranged into two groups, those that have to be joined to a main verb and those that can either stand alone or help a main verb.
Auxiliary verbs that can also be main verbs: be, being, been; am, is, are, was, were; have, has, had; do, does, did.
Auxiliary verbs that must appear with main verbs: can, could; may, might; shall, should; will, would; must, ought (to).