The verbs will, would, shall, should, can, could, may, might, must, ought to and used to are called modal verbs, because they convey a particular meaning or mood. Modal verbs have some important grammatical features.
Apart from can and could they do not have past or future forms, or use any auxiliary verb (e.g. do/did) either to make a question or a negative:
Will she be there? I couldn't help it.
An exception to this is used to, which often takes did as an alternative:
Did you use to be good at maths? Used you to be good at maths?
Modal verbs (except ought to and used to) take the infinitive without to:
They can't have arrived yet. You must have seen her.
An alternative to can is be able to, which is used for all tenses:
They'll be able to book the tickets soon.
Could is used as the Past Simple or conditional of can.
An alternative to must is have to, which is used for all tenses: Surely you didn't have to pay!
Dare and need can be used either as normal verbs:
I didn't dare (to) mention it. He doesn't need to borrow any money.
or as modal verbs: She daren't tell him the truth. You needn't cry.
Exercise 1. Complete the sentences with the correct form of the following verbs:
Will dare may be able ought should used have
How _____ you speak to me like that?
He really _____admit that he is to blame for the whole affair.
When she was younger, she never _____ to wear make-up.
_____ I use your phone, please?
Do you think he'll _____ to come up with the money in time.
Steel yourself - it's just possible you might _____ to pay a heavy fine.
The authorities _____ to have investigated the incident more thoroughly.
There's every likelihood our candidate _____ win the election.
Exercise 2. Correct the sentences if necessary.
Did you must renew your visa last February?
Your accountant ought to have give you better advice.
The President can't to be re-elected for a fourth term.
You could come on the expedition if you wanted to.
I might had returned the book. I just can't remember.
Will you can help me with my luggage if I tell you exactly when I'm leaving?
They didn't could have a picnic because of the inclement weather.
The police must have preferred charges against him by now.
Must/mustn't are used
a to express commands or the authority of the speaker: You must do what I say in future.
b to express an internal obligation, from the speaker's viewpoint:
I mustn't forget to phone Jake this evening.
Have got to (have to) can be used in all tenses instead of must to express external obligation, especially rules or laws made by someone else:
We've got to check in at least an hour before take-off.
The modal form needn't is usually used to express the speaker's own opinion or viewpoint:
You needn't phone me unless there's any problem.
Don't need to do and didn't have to do mean that something was not done, because it was not necessary. Needn't have done means that something was done, although it was not necessary.
Exercise 3. Complete the sentences, using the correct word or phrase from the pair in brackets.
You _____ take flowers to your hostess if you don't want to. (mustn't/needn't)
She _____ do all the work herself, because nobody will help her. (must/has to)
I'm so tired, I really _____ go to bed now. (must/have to)
What a pity you _____ go now! You'll just catch your train if you hurry. (must/have to)
Yesterday all the students _____ read a poem aloud. (must/had to)
I was going to book the tickets, but Angus bought them on his way home, so I _____. (didn't need to/needn't have done)
When I turned up at the hospital and saw Natalia surrounded by visitors, I realized I _____ coming. (didn't need to bother/ needn't have bothered)
You _____ smoke in here. It's against the rules. (mustn't/needn't)
Should/shouldn't and ought to/oughtn't to are both used
a to express the idea of duty or strong advise. Should gives the speaker's subjective opinion, whereas ought to is a little more forceful, based on objective considerations.
Other uses of should are
b instead of would in formal writing: I should be glad to hear from you as soon as possible.
c to convey an assumption: Jane should have arrived at the office by now.
d as a formal or literary alternative to an if clause:
Should the star be ill, the concert will be cancelled.
to express a remote possibility with an if clause:
If they should ever ask about it, tell them the truth.
and after in case: We'll hide, in case they should see us.
e to make a suggestion or a request for advice: What should I wear to the christening?
f after so that, in order that, verbs like command, insist, suggest, and adjectives like vital, essential, necessary, anxious, to show that something must be done:
I'm anxious that he should feel at home.
Exercise 4. Match the different uses of should with the categories above.
Should there be any difficulty, ring me at home.
Do you think I should put in an application or not?
I took a book with me, in case I should have to wait.
The irate customer insisted that the manager should apologize.
By this time my brother should be at the station.
You really should start planning for your future.
If anyone should inquire, kindly take their name and address.
It's essential that this should be kept in the strictest confidence.
Could is used to express general ability in the past: I could read when I was three.
For an achievement on a particular occasion in the past, was able to/managed to/succeeded in are used instead: How many letters were you able to write last night?
Couldn't is used for both general and particular ability.
Used to and would are both used to talk about past habits. Used to is used for situations and actions, whereas would only refers to repeated actions: I used to have a guinea-pig when I was younger. Lisa would always eat porridge for breakfast.
Exercise 5. Complete the sentences with could, couldn't, was/were able to, used to or would.
Unfortunately my cousin _____ see his solicitor that day, as no appointment had been made.
When I was a child, I _____ always take the same route to school and back.
The boy next door _____ swim when he was five, but he never learnt to sail.
In the past people _____ have larger families than they do nowadays.
Stephen had to give up half-way round, and _____ finish the marathon.
Despite the thunderstorm we _____ climb right to the top of the mountain.
I sat on the terrace, drinking coffee. From there I _____ see the vineyards below me.