Learning English Grammar | auxiliary verbs do, be, have

How to use auxiliary verbs correctly in English

Why they think this? [incorrect]
Why do they think this? [correct]

I not agree with this idea. [incorrect
I do not agree with this idea. [correct]

Do, be and have are called auxiliary verbs.
This means that they help to change the main verb.

Do is used with the infinitive to make negatives or questions:
I don't agree. (not I not agree)
Do you agree? (not Are you agree?)

Do can be used with other question words:
Why do you agree? (not Why you agree?)

Be is used with -ing verb forms to make the continuous tenses:
I am studying English in Cambridge. (not I studying English)

Have is used with the past participle of the verb to make the perfect tenses:
I have been here for three months. (not I am been here or I been here)

The auxiliary verb must agree with the subject of the verb:
Does your mother drink tea? (not Do your mother drink tea?)

We make questions and negatives without do if another auxiliary verb is used:
Are you studying here? (not Do you are studying here?) (auxiliary verb be)

See more English grammar mistakes