These are the verbs be, do, and have. Note that we can use these three verbs as helping verbs or as main verbs. On this page we talk about them as helping verbs. We use them in the following cases:
1. be - to make continuous tenses (He is watching TV.) / to make the passive (Small fish are eaten by big fish.)

2. have - to make perfect tenses (I have finished my homework.)

3. do - to make negatives (I do not like you.) / to ask questions (Do you want some coffee?) / to show emphasis (I do want you to pass your exam.) / to stand for a main verb in some constructions (He speaks faster than she does.)