modal auxiliary verbs
an auxiliary verb that is used with another verb to indicate its mood, as can, could, may, might, must, shall, should, will, and would: it has no special form in the third person singular and no present or past participle
She should be here by now.
I could swim quite well when I was younger.
You might have discussed it with me first.
Could you open the window please?
Must you make so much noise?
He had to take his brother.
We ought to be going.
It is clear from examples that the modal verb occupies the first position in verb phrase, coming before any other auxiliary verb (like have or be) and the main lexical verb.
The subject of the sentence has no effect on the form of the modal since almost in all cases they do not change at all.
Below lists the main modal auxiliaries that that divided into two categories pure modals and semi-modals.
Pure modals
Semi-modals
can ought to
could has/have (got) to
may be able to
might
shall
should
will
would
need ***
The forms of pure modals
they never change their form irrespective of the subject of the sentence
e.g. he can play, not *he cans play
following on from the above feature, they do not change to show past tense
e.g. she had to leave not *she musted leave
they all carry the negative of the sentence by the addition of not/n’t
e.g. I can’t remember not *I don’t can remember
they all form questions by inversion with the subject of the sentence.
e.g. should I go?
they are all followed by the base form of the verb without the addition of to
e.g. he can play not *he can to play
The forms of semi-modals
Please look at the form of each individual semi-modal.
Be able to
We use this semi-modal to express possibility or the ability to do something.For example:
Positive: I am able to walk.
Negative: I am not able to walk.
Yes/no question: Are you able to walk? .
Has/have (got) to
it is necessary shows obligation and giving advice.
have to/has to –in the present/near future
had to –in the past
will have to –in the future
You have to be more punctual.
She has to take responsibility for the event.
I had to help my brother build his house.
You don’t have to come if you don’t want to.
She didn’t have to do all the shopping.
From these few examples it should be clear that the negative not again attaches itself to the auxiliary verb (modal or main) that comes immediately after the subject of the sentence.
Ought to
Ought to usually has the same meaning as should, particularly in affirmative statements in the present:
You should/ought to get your hair cut.