There are auxiliary and main verbs in English.
Main verbs express a dynamic or abstract action like to write, to run, to think. If we take the main verb away from the sentence, it will lose its meaning.
Auxiliary verbs are helping verbs, they don't have their own meaning. We use them together with a main verb to make positive, negative and question sentences in various tenses.
There are three helping verbs:
- be (am, are, is, was, were, being, been) — used in continuous tenses
- do (does, did) — used in simple tenses
- have (has, had, having) — used in perfect tenses
The verbs "be", "do", "have" can act as main verbs and as auxiliaries.
You are smart. "be" is the main verb
What are you doing? "be" is the auxiliary verb in the present continuous
I have a car. "have" is the main verb
I have finished the letter. "have" is the auxiliary verb in the present perfect
She does a lot of exercises. "does" is the main verb
She doesn't like high-heeled shoes. "doesn't" is the auxiliary verb in the present simple
Modal verbs are a separate group of auxiliary verbs. Can, could, may, might, must, shall, should, will, would are modal verbs.
When we answer a yes/no question, sometimes we don't want to repeat the whole sentence. We can use the auxiliary verb to make a short answer.
Are you leaving? Yes, I am. (= Yes, I am leaving.)
Has he been sleeping? Yes, he has. (= Yes, he has been sleeping.)
Did you go to the cinema yesterday? No, I didn't. (= No, I didn't go to the cinema yesterday.)
May I sit here? Yes, you may. (Yes, you may sit here.)