Modal Verbs

Can/ Be able to / Can’t
CAN
Different uses:
Abilities or capacities (to know or to be able to).
Mary can swim very fast.
Request, ask or give permission
Can you call me tonight.
Possibility
I can meet you later.
It can also be used for suggestions.
You can eat ravioli if you like pasta.

Be able to
It expresses abilities like can and it is used in all the verbal tenses where can is not used.
I was able to finish my homework on time
Can’t
Different uses:
Impossibility in the present
Mary can’t swim very fast
Lack of ability (not to know) or capacity (not to be able to): I can’t eat a whole cake by myself
Prohibition
You can’t drive without a licence
Disbelief
That can’t be the price – it’s much too cheap.

Could
It is the past of can and it is used to express:
Ability or capacity in the past
She could run fast when she was a child
Polite request
Could you help me with these suitcases?
Polite suggestion
You could exercise and eat healthier food
Possibility –less probable than with can-
Mark could join us the cinema.

May/ might
Both of them express possibility, but might is more remote.
It may/ might rain tomorrow
In questions, may is the polite way of asking for things.
May I have a coffee, please?

Would
In questions, it is a formal way of asking for things.
Would you open the window, please?
With the verb “like” is used to make offers and invitations.
Would you like something to drink?

Must / Have to
Both express obligation, but must is only used in the present and have to in the other tenses. Authority people use must, while have to is used by everybody.
You must bring your books to class
I have to buy the tickets today.
Must is also used to express a logical deduction about present fact.
She’s got a great job. She must be very happy.

Need to / Needn’t
Need to is not a modal, but it is used in affirmative sentences, like have to, to express obligation and necessity.
I need to cook dinner tonight.
Needn’t, on the contrary, is a modal and indicates lack of oblication and necessity, like don’t have to
You needn’t bring anything to the party.

Musn’t / Don’t have to
Musn’t shows prohibition.
You musn’t exceed the speed limit
Don’t have to means not have to, i.e., lack of obligation and necessity, like needn’t
I don’t have to get up early tomorrow

Should /Ought to
Both of them express advise or opinion, but should is used more frequently, since ought to is quite strange in negative and interrogative.
You should/ought to improve your pronunciation

Shall
It is used in the interrogative to offer oneself to do something and to make a suggestion.
Shall I help you with your luggage?

MODAL PERFECTS
Must have + participle
It expresses a logical conclusion about a past fact.
Rob has arrived late. He must have been in a traffic jam.
May/might have + participle
We use it to make a supposition about something in the past.
She may/might have taken the wrong bus.

Could have + participle
Ability to do something in the past which in the end was not done
You could have asked the doctor before taking the medicine.
Couldn’t have + participle
Certainty that something did not happen
He couldn’t have gone to the concert because he was doing the test.

Would have + participle
Desire to do something in the past which in fact could not be done.
I would have gone to the party, but I was too busy.
Should/ought to + participle
Criticism or regret after an event
You should/ought to have warned me earlier
Shouldn’t have + participle
Criticism or regret after an event, showing that it shouldn’t have happened
He shouldn’t have forgotten about her birthday

Needn’t have + participle
An unnecessary past action
You needn’t have brought anything to my party.