Basically, auxiliary verbs are function words, a type of closed class which is constituted of words that have a grammatical function as opposed to content words, which are an open class of lexical words. An auxiliary verb is used to add functional or grammatical content to the information expressed by another verb, considered to be the main verb. Auxiliary verbs are also called helping verbs
Examples:am | is | are |
was | were | |
has | have | had |
do | does | did |
will | would | |
shall | should | |
can | could | |
may | might | must |
Be | Been | Being |