The verb to be.
The verb to do.
The verb to have.
Every sentence must have a verb.
To depict achievable activities, writers use action verbs.
To describe conditions, writers choose linking verbs.
Be, do, and have are both stand-alone verbs and auxiliary verbs. When these verbs are auxiliary, you will find them teamed with other verbs to complete the verb phrase.
Below are the auxiliary verbs. You can conjugate be, do, and have; the modal auxiliaries, however, never change form.
Be - am, is, are, was, were, being, been.
Do - does, do, did.
Have - has, had, having.
The modal auxiliaries:
can, could, may, might, must, ought to, shall, should, will, would.