The verbs be (am, is, was), have and do, when used with ordinary verbs to make tenses, passive forms, questions and negatives, are called Auxiliaries. An auxiliary verb adds functional or grammatical meaning to the clause in which it appears. It determines the mood, tense or aspect of another verb in a verb phrase. Auxiliary verbs are also known as helping verbs, and they always precede main verbs within a verb phrase.
There are two types of auxiliaries: primary auxiliaries and modals. Some basic characteristics of primary auxiliaries are that these can be used as principal or main verbs; these can change their forms according to the number and person of the subject; these can form tenses. The verb ‘be’ forms voices as well.
There are a few differences between modals and main verbs: modals do not take word endings to form participles or agree with their subject. This means that we say ‘He may go to the market’, but never, ‘He mays go to the market.’ Also, modals come before ‘not’ in negative clauses; whereas, main verbs follow ‘not’ to form negatives.