The main modal verbs (or modals) are:
can, may, will, shall, must, could, might, would, should
Ought to, used to, dare and need are also used as modal verbs, but they have other uses as well.
WHEN TO USE MODAL VERBS
Modals have several meanings, so you need to think about the meaning of the sentence as a whole to be sure that your choice of modal expresses exactly what you want to say. The main ideas that modals are used to express are shown in the following sections.
(=”allowing” someone to do something)
If you want to give or ask for permission, use can or may. May is more polite or formal than can.
You can leave when the bell rings.
Customers may purchase extra copies at half price.
Could is a polite way of asking for permission.
Could I leave early today?
Might is a very formal and old-fashioned way of asking for permission.
Might I borrow your umbrella?
(=”saying” what someone must do)
If you want to demand that something happens, or that someone does something (=”to” express obligation), use must. You can also use this idea about yourself, in order to express a sense of duty.
The builders must finish the job today.
We mustn’t leave the house before 6 o’clock.
I must remember to bring my notebook.
(=”saying” what you are going to do)
If you want to say that you intend to do something, use will or shall. You can emphasize the meaning of intention if you say the modal louder than the surrounding words.
Shall is only used with the first person (I or we), and is much less common than will. It is hardly ever used in American English.
This letter says they will definitely give us our money back.
I shan’t stay long.
To express an intention at a time in the past, use would.
I tried to explain, but nobody would listen.
Use would if there are conditions controlling whether something will take place.
I would leave tomorrow, if I had the money.
(=”saying” whether you are able to do something)
If you want to say whether someone is able to carry out an action, use can.
Guy can speak Russian.
Can you remember her name?
I can’t find my shoes!
When you put these sentences into the past tense, use could.
He was late for school because he couldn’t find his bag.
Use could if there are conditions controlling whether the event will take place.
I could leave tomorrow, if I had the money.
(=”saying” whether something is possible)
If you want to say that something is possible, use can or may. May is more polite or formal than can.
You can go by bus from London to Liverpool.
You may find the manager is still there, if you go to the office now.
If you want to suggest that the action is less likely to happen, use could or might. If you use might, you mean that the action is especially unlikely.
We could go by bus.
We might go by bus. (=”it” is possible, but only if there are no problems)
(=”saying” whether something is likely)
If you want to suggest that an event is likely to happen, use should or ought to. It will probably take place, but you are not completely sure.
They should have had our reply by now.
If you take these tablets, you should be all right.
We ought to be there by 6 o’clock.
(=”saying” that something is the right thing to do)
If you want to say that you think it is a good thing for something to happen, use should or ought to. If you think that it is a bad thing for something to happen, put these verbs into the negative.
You should get the early flight, if you want to be in good time.
You ought to see the doctor as soon as possible.
You shouldn’t say things like that.
You oughtn’t to have left the engine running.
(=”saying” that something is necessary)
If you want to say that it is necessary for something to happen, use must.
We really must go now.
I must get my hair cut this weekend.
If you want to express the opposite meaning (=”it” is unnecessary for something to happen) use needn’t/need not or not need to.
There’s plenty of time, so you needn’t worry.
Don’t use mustn’t because this gives the meaning of obligation.
(=”saying” that you are sure about something)
If you want to say that you are sure something is true, use must.
You must be tired, after all your hard work
They must have left by now.
To express the opposite meaning (=”you” are sure something is not true) use can’t.
You can’t be that tired! you’ve only been working for an hour!
They can’t have left yet.
(=”saying” what you think is going to happen)
If you want to say that something is certain to happen, use either will or shall. As with the other uses of these words, shall tends to be found only with the first person (I or we), and is much less common than will. Shall is very rare in American English.
The cars will be there on time, I promise.
There is no doubt that we shall win.
HOW TO USE MODAL VERBS
– Modal verbs are used with the basic form of the verb (=”the” infinitive form, without to).
You must pay now. NOT You must to pay now.
They can go home if they want. NOT They can to go home if they want.
– Modal verbs do not have an -s ending in the present tense of the third person singular,
He can speak French. NOT He cans speak French.
– Modal verbs do not use do in questions or negatives.
Can you remember her name? NOT Do you can remember her name?
We must not be late. NOT We don’t must be late.
Should we lock the door? NOT Do we should lock the door?
– Modal verbs do not have an infinitive, a past participle, or a present participle.
– In spoken English, you often use short forms when you use the modal verb in the negative.
could not couldn’t
will not won’t
must not mustn’t
shall not shan’t
might not mightn’t
would not wouldn’t
should not shouldn’t
ought not oughtn’t
Mustn’t, shan’t, mightn’t, and oughtn’t are normal in British English, but American speakers usually say must not, shall not, might not, and ought not.