The modal auxiliaries (or modals) include the following:
can, could, may, might, must, should, will, would, . . .
Modals are always followed by the base form of a verb or auxiliary verb.
Modals are always the same form no matter what the subject is.
In standard American English, a predicate verb phrase cannot contain more than one modal.
correct
He will be able to go.
not correct
* He will can go.
Below are example sentences containing the modal may and the verb go. Notice that the form of the modal does not change. Also notice that the base form of a verb or auxiliary verb always follows the modal.
I may go.
You may go.
He may go.
It may go.
We may go.
They may go.
He may have gone.
They may have gone.
He may be going.
They may be going.
He may have been going.
They may have been going.
Modals and related verb phrases add meanings to verbs. Below are some of those meanings:

Ability/Availability
future: will be able to
present: can, am/is/are able to
past: could, was/were able to
Requests
present/future: can, could, will, would
Permission
future: will be allowed to
present/future: may, can, could, am/is/are allowed to
past: could, was/were allowed to
Possibility
present/future: may, might, could
past: may have, might have, could have
Impossibility
present/future: couldn’t, can’t
past: couldn’t have
Advisability
present/future: should, ought to, had better
past: should have, ought to have, had better have
Expectation
present/future: should, ought to
past: should have, ought to have
Necessity
future: will have to
present/future: must, have to, has to
past: had to
Lack of Necessity
future: won’t have to
present/future: don’t have to, doesn’t have to
past: didn’t have to
Prohibition
present/future: must not, may not, cannot
past: could not
Logical Deduction (=Probability)
present: must, have to, has to
past: must have, have to have, has to have
SOCIAL MODALS
The choice of modal depends partly on the social situation.
We often use formal language with strangers (people we don’t know) and superiors (people with some power over us such as our employers, doctors, and teachers).
We often use informal language with our equals (our friends and family) and subordinates (people we have some power over such as our employees or children).
General requests (present and/or future):
Will you help me? (Informal Are you willing?)
Would you help me (Formal Are you willing?)
Can you help me? (Informal Are you able?)
Could you help me (Formal Are you able?)
Requests for permission (present and/or future):
May I leave the room? (Formal)
Might I leave the room? (Formal rarely used)
Could I leave the room? (Less formal
Can I leave the room? (Informal)
Expressing suggestions, advice, warnings, necessity (present and/or future):
The choice of modal depends partly on the urgency of the message or the authority of the speaker/writer or both.
Suggestions:
You could see the doctor.
You might see the doctor.
Advice:
You should see the doctor.
You ought to see the doctor.
Warning/strong advice:
You had better see the doctor. 
Strong advice/necessity:
You have to see the doctor.
You have got to see the doctor.
You must see the doctor.
No choice:
You will see the doctor.
MODALS OF BELIEF (beliefs about present time)
The choice of modal depends partly on what the speaker or writer believes.
Someone is knocking at the door.
That could be Fred.
That might be Fred.
= It’s possible. I’m less than 50% sure.
That may be Fred.
= It’s possible. I’m less than 60% sure.
That should be Fred.
That ought to be Fred.
= I’m expecting Fred and I think he’s here.
That must be Fred.
That has to be Fred.
That has got to be Fred.
= It’s probably Fred. I have a good reason to believe it is Fred.
That will be Fred.
= I believe it is Fred. I’m about 99% sure.
That can’t be Mary.
That couldn’t be Mary.
= It’s impossible. I’m about 99% sure.
That is Fred.
= I know it’s Fred. I’m 100% sure.
MODALS OF BELIEF (beliefs about past time)
The choice of modal depends partly on what the speaker or writer believes.
Someone was knocking at the door.
That could have been Fred.
That might have been Fred.
= It’s possible. I’m less than 50% sure.
That may have been Fred. 
= It’s possible. I’m less than 60% sure.
That must have been Fred.
That has to have been Fred.
That has got to have been Fred.
= It was probably Fred. I have a good reason to believe it was Fred.
That couldn’t have been Mary.
= It’s impossible. I’m about 99% sure.
That was Fred.
= I know it was Fred. I’m 100% sure.
MODALS OF BELIEF (beliefs about future time) The choice of modal depends partly on what the speaker or writer believes.
What will the weather be like tomorrow?
It could rain tomorrow.
It might rain tomorrow.
= It’s possible. I’m less than 50% sure.
It may rain tomorrow.
= It’s possible. I’m less than 60% sure.
It should rain tomorrow.
It ought to rain tomorrow.
= I expect it will rain.
It will rain tomorrow.
= I believe it is going to rain. I’m about 99% sure.
It couldn’t snow tomorrow.
= It’s impossible. I’m about 99% sure.



Sumber: faculty.deanza.edu


http://myzeroline.blogspot.com/