Auxiliary verbs, or helping verbs, are usually used together with a main verb to add extra meaning to a sentence. They can also be used to conjugate verbs in compound tenses, to ask and answer yes-no questions or to express negation and the passive voice.
The three most common auxiliary verbs, “be”, “have” and “do”, are called primary auxiliaries.
Like ordinary verbs, they have both infinitive and irregular conjugated forms. They can sometimes be used alone as main verbs, e.g.
“He does just as I say” (to do), “I had everything I needed” (to have), “We have never been to England” (to be).
As an auxiliary, “be” is mainly used for continuous forms, e.g. “I am learning English” (to learn) and for the passive voice, e.g.
“She was raised by her mother” (to raise).
“Have” is also used as an auxiliary to form compound tenses such as the present or the past perfect, both in the active and the passive voice.
E.g. “Have you tried to call him?” (to try), “He had been given a second chance” (to give).
“Do” is used as an auxiliary to form questions and negative sentences with ordinary verbs. In the spoken negative form,
“not” is contracted and changes to “-n’t”, e.g. “Did you see that? – No, I did not / didn’t!” (to see).
“Do” can also be used for emphasis with ordinary verbs, e.g. “Stop calling me lazy, I did clean the bathroom!” (to clean)
Modal verbs are a special type of auxiliary verbs. They are always followed by the infinitive of a main verb without “to”, e.g. “You can leave now” (to leave), “She might know about this already” (to know).
They have only one form and cannot be conjugated with other auxiliaries (no continuous tenses or passive forms): you cannot say “she musts”, “I am mighting” or “you were coulded”. Negatives are formed by adding “not” after the modal verb. Some modals can be contracted: “can’t”, “couldn’t”, “wouldn’t”, “shouldn’t”, “won’t” and “shan’t”.
Two modals can be used as past forms of others: can (present) could (past) and will (present, future) would (past).
Actually I do know that, but thanks!
be able to/am able to/are able to/is able to/was able to/were able to
I can read pretty fast.
She was not able to catch her plane.
Could you finish the test yesterday?
Can I leave now?
May I come in?
Could you close the window?
Might I join you?
be not to
You can't walk on the grass.
He must not fight with his sister.
You may not smoke in this room.
We are not to travel alone.
Obligation / Order
have to/have got to
We must stay quiet.
You have to go now.
I am to stay here and wait for her.
You will stop answering back now!
You shall work harder next time.
Duty / Advice
He should study more seriously.
You ought go to bed earlier.
You must see this movie once in your life!
have to/has to/had to
need to/needs to/needed to
He must stop complaining constantly.
I have to catch my train at 8 am.
I need to pay the rent every month.
Absence of necessity
do not have to/don't have to/does not have to/doesn't have to/did not have to/didn't have to
need not/needn't/needs not/do not need to/don't need to/does not ned to/doesn't need to/did not need to/didn't need to/needed not
You don't have to come if you don't want to.
You needn't worry, he will be fine.
I'll never lie to her again.
I shall honour this promise.
Request / Invitation
Shall we dance?
Will you come to the party tonight?
Would you please help me?
This can't be right.
He couldn't stop crying when he heard the news.
Regrets won't help you now!
I told you it just wouldn't work.
That'll be him ringing the doorbell.
She should be home by now.
They must be very proud.
He can't be more than thirty.
He could be dead already.
It may or may not rain for the picnic.
This might be the answer to my problem.
It can be that she didn't want to bother you.
I used to play in this park when I was little.
For years, she would visit her friends every Sunday.
do not dare/don't dare/does not dare/doesn't dare/daren't
did not dare/didn't dare/dared not
How dare you call me selfish?
I don't dare look down from the top.
She dared not complain about her bully.