Can, could and (be) able to
A. We use can to say that something is possible or that somebody has the ability to do something.
* We use can + infinitive (can do/can see etc.):
* We can see the lake from our bedroom window.
* Can you speak any foreign languages?
* I can come and see you tomorrow if you like.
The negative is can't (= cannot):
* I'm afraid I can't come to the party on Friday.
B. (Be) able to ... is possible instead of can, but can is more usual:
* Are you able to speak any foreign languages?
But can has only two forms, can (present) and could (past). So sometimes it is necessary to use (be) able
to... Compare:
* I can't sleep.
but I haven't been able to sleep recently. (can has no present perfect)
* Tom can come tomorrow.
but Tom might be able to come tomorrow. (can has no infinitive)
C. Could and was able to...
Sometimes could is the past of can. We use could especially with:
see hear smell taste feel remember understand
* When we went into the house, we could smell burning.
* She spoke in a very low voice, but I could understand what she said.
We also use could to say that somebody had the general ability or permission to do something: * My grandfather could speak five languages.
* We were completely free. We could do what we wanted. (= we were allowed to do ...) We use could for general ability. But if we are talking about what happened in a particular situation, we use
was/were able to... or managed to... (not could):
* The fire spread through the building quickly but everybody was able to escape.
or ... everybody managed to escape. (but not 'could escape')
* They didn't want to come with us at first but we managed to persuade them.
or ... we were able to persuade them. (but not 'could persuade')
Compare:
* Jack was an excellent tennis player. He could beat anybody. he had the general ability to beat
anybody)
but* Jack and Alf had a game of tennis yesterday. Alf played very well but in the end Jack managed to beat
him. or ... was able to beat him. (= he managed to beat him in this particular game)
The negative couldn't (could not) is possible in all situations:
* My grandfather couldn't (could not) is possible in all situations
* We tried hard but we couldn't persuade them to come with us.
* Alf played well but he couldn't beat Jack.
Must and can't
A. Study this example:
We use must to say that we feel sure something is true:
* You've been travelling all day. You must be tired. (Travelling is tiring and you've been travelling all day, so
you must be tired.)
* 'Jim is a hard worker.' 'Jim? A hard worker? You must be joking. He's very lazy.'
* Carol must get very bored in her job. She does the same thing every day.
We use can't to say that we feel sure something is not possible:
* You've just had lunch. You can't be hungry already. (People are not normally hungry just after eating a
meal. You've just eaten, so you can't be hungry.)
* Brian said he would definitely be here before 9.30. It's 10 o'clock now and he's never late. He can't be
coming.
* They haven't lived here for very long. They can't know many people.
Study the structure:
I/you/he (etc.) must/can't be (tired/hungry/at work etc.)
I/you/he (etc.) must/can't be (doing/coming/joking etc.) do/go/know/have etc.
I/you/he (etc.) must/can't do/go/know/have etc.
B. For the past we use must have (done) and can't have (done). Study this example:
George is outside his friends' house.
He has rung the doorbell three times but nobody has answered.
They must have gone out. (otherwise they would have answered)
* The phone rang but I didn't hear it. I must have been asleep.
* I've lost one of my gloves. I must have dropped it somewhere.
* Jane walked past me without speaking. She can't have seen me.
* Tom walked straight into a wall. He can't have been looking where he was going. Study the structure:
I/you/he (etc.) must/can't have been (asleep/at work etc.)
I/you/he (etc.) must/can't have been (doing/working etc.)
I/you/he (etc.) must/can't have done /gone/known/had etc.
Couldn't have ... is possible instead of can't have...:
* She couldn't have seen me.
* Tom couldn't have been looking where he was going.
May and might (1)
A. Study this example situation:
You are looking for Bob. Nobody is sure where he is but you get some suggestions.
You: Where's Bob?
He may be in his office. (= perhaps he is in his office)
He might be having lunch. (= perhaps he is having lunch)
Ask Ann. She might know. (= perhaps she knows)
We use may or might to say that something is a possibility. Usually you can use may or might you can say:
* It may be true. or It might be true. (= perhaps it is true)
* She might know. or She may know.
The negative forms are may not and might not (or mightn't):
* It might not be true. (= perhaps it isn't true)
* I'm not sure whether I can lend you any money. I may not have enough. (= perhaps I don't have enough)
Study the structure:
I/you/he (etc.) may/might (not) be (true/in his office etc.)
I/you/he (etc.) may/might (not) be (doing/working/having etc.)
I/you/he (etc.) may/might (not) do/know/have/want etc.
B. For the past we use may have (done) or might have (done):
* A: I wonder why Kay didn't answer the phone.
B: She may have been asleep. (= perhaps she was asleep)
* A: I can't find my bag anywhere.
B: You might have left it in the shop. (= perhaps you left it in the shop)
* A: I was surprised that Sarah wasn't at the meeting.
B: She might not have known about it. (= perhaps she didn't know)
* A: I wonder why Colin was in such a bad mood yesterday.
B: He may not have been feeling well. (= perhaps he wasn't feeling well)
Must and have to
* Oh, it's later than I thought. I must go. or I have to go.
But there is a difference between must and have to and sometimes this is important:
Must is personal. We use must when we give our personal feelings.
'You must do something' = 'I (the speaker) say it is necessary':
* She's a really nice person. You must meet her. (= I say this is necessary)
* I haven't phoned Ann for ages. I must phone her tonight.
Compare:
* I must get up early tomorrow. There are a lot of things I want to do.
Have to is impersonal. We use have to for facts, not for our personal feelings.
'You have to do something' because of a rule or the situation:
* You can't turn right here. You have to turn left. (because of the traffic system)
* My eyesight isn't very good. I have to wear glasses for reading.
* George can't come out with us this evening. He has to work.
* I have to get up early tomorrow. I'm going away and my train leaves at 7.30.
If you are not sure which to use, it is usually safer to use have to.
B. you can use must to talk about the present or future, but not the past:
* We must go now.
* We must go tomorrow. (but not 'We must go yesterday')
You can use have to in all forms. For example:
* I had to go to hospital. (past)
* Have you ever had to go to hospital? (present perfect)
* I might have to go to hospital. (infinitive after might)
In questions and negative sentences with have to, we normally use do/does/did:
* What do I have to do to get a driving licence? (not 'What have I to do?')
* Why did you have to go to hospital?
* Karen doesn't have to work on Saturdays.
C. Mustn't and don't have to are completely different:
You mustn't do something = it is necessary that you do not do it (so, don't do it):
* You must keep it a secret. You mustn't tell anyone. (= don't tell anyone)
* I promised I would be on time. I mustn't be late. (= I must be on time)
You don't have to do something = you don't need to do it (but you can if you want):
* You can tell me if you want but you don't have to tell me. (= you don't need to tell me)
* I'm not working tomorrow, so I don't have to get up early.
D. You can use 'have got to' instead of 'have to'. So you can say:
* I've got to work tomorrow. or have to work tomorrow.
Must mustn't needn't
A. Must mustn't needn't
'You must do something' = it is necessary that you do it:
* Don't tell anybody what I said. You must keep it a secret.
* We haven't got much time. We must hurry.
'You mustn't do something' = it is necessary that you do not do it (so don't do it):
* You must keep it a secret. You mustn't tell anybody else. (= don't tell anybody else)
* It's essential that nobody hears us. We mustn't make any noise.
'You needn't do something' = it is not necessary that you do it, you don't need to do it:
* You can come with me if you like but you needn't come if you don't want to. (= it is not necessary for you
to come)
* We've got plenty of time. We needn't hurry. (= it is not necessary to hurry)
B. Instead of needn't, you can use don't/doesn't need to. So you can say:
* We needn't hurry. or We don't need to hurry.
Remember that we say 'don't need to do', but 'needn't do' (without to).
Needn't and don't need to are similar to don't have to (see Unit 31C):
* We've got plenty of time. We don't have to hurry.
C. Needn't have (done)
Study this example situation:
I think it's going to rain. I'll take the umbrella.
I needn't have brought the umbrella.
George had to go out. He thought it was going to rain, so he decided to take the umbrella.
But it didn't rain, so the umbrella was not necessary. So:
He needn't have taken the umbrella. 'He needn't have taken the umbrella' = He took the umbrella but this was not necessary. Of course, he
didn't know this when he went out.
Compare needn't (do) and needn't have (done):
* That shirt isn't dirty. You needn't wash it.
* Why did you wash that shirt? It wasn't dirty. You needn't have washed it.
D. Didn't need to (do) and needn't have (done)
I didn't need to ... = it was not necessary for me to... (and I knew this at the time):
* I didn't need to get up early, so I didn't.
* I didn't need to get up early, but it was a lovely morning, so I did.
'I needn't have (done) something' = I did something but now I know that it was not necessary:
* I got up very early because I had to get ready to go away. But in fact it didn't take me long to get ready.
So, I needn't have got up so early. I could have stayed in bed longer.
Should (1)
A. You should do something = it Is a good thing to do or the right thing to do. You can use should to give
advice or to give an opinion:
* You look tired. You should go to bed.
* The government should do more to help homeless people.
* 'Should we invite Susan to the party?' 'Yes, I think we should.'
We often use should with I think/I don't think/Do you think...?:
* I think the government should do more to help homeless people.
* I don't think you should work so hard.
* 'Do you think I should apply for this job?' 'Yes, I think you should.'
'You shouldn't do something' = it isn't a good thing to do:
* You shouldn't believe everything you read in the newspapers.
Should is not as strong as must:
* You should apologise. (= it would be a good thing to do)
* You must apologise. (= you have no alternative)
B. We also use should when something is not right or what we expect. For example:
* I wonder where Liz is. She should be here by now. (= she isn't here yet, and this is not normal)
* The price on this packet is wrong. It should be L1.20, not L1.50. * Those boys shouldn't be playing football at this time. They should be at school.
We use should to say that we expect something to happen:
* She's been studying hard for the exam, so she should pass. (= I expect her to pass)
* There are plenty of hotels in the town. It shouldn't be difficult to find somewhere to stay. (= I don't expect
that it will be difficult)
C. 'You should have done something' = you didn't do it but it would have been the right thing to do:
* It was a great party last night. You should have come. Why didn't you? (= you didn't come but it would
have been good to come)
* I'm feeling sick. I shouldn't have eaten so much chocolate. (= I ate too much chocolate)
* I wonder why they're so late. They should have been here an hour ago.
* She shouldn't have been listening to our conversation. It was private.
Compare should (do) and should have (done):
* You look tired. You should go to bed now.
* You went to bed very late last night. You should have gone to bed earlier.
D. Ought to ...
You can use ought to instead of should in the sentences on this page. Note that we say 'ought to do...'
(with to):
* Do you think I ought to apply for this job? (= Do you think I should apply ...?)
* Jack ought not to go to bed so late. (= Jack shouldn't go ...)
* It was a great party last night. You ought to have come.
* She's been studying hard for the exam, so she ought to pass.
Should (2)
A. You can use should after a number of verbs, especially:
suggest propose recommend insist demand
* They insisted that we should have dinner with them. * I demanded that he should apologise.
* What do you suggest I should do?
In the same way, you can use should after suggestion/proposal/recommendation etc.
* What do you think of Jane's suggestion that I should buy a car?
and also after 'it's important/vital/necessary/essential that ...'
* It's essential that you should be here on time.
B. You can also leave out should in all the sentences in Section A:
* It's essential that you be here on time. (= that you should be here)
* I demanded that he apologise.
* What do you suggest I do?
This form (you be/he apologise etc.) is sometimes called the subjunctive.
You can also use normal present and past tenses:
* It's essential that you are here on time.
* I demanded that he apologised.
Be careful with suggest. You cannot use to ... ('to do/to buy' etc.) after suggest:
* What do you suggest we should do?
or What do you suggest we do? (but not 'What do you suggest us to do?')
* Jane suggested that I (should) buy a car.
or Jane suggested that I bought a car. (but not 'Jane suggested me to buy')
For suggest ~ing, see Unit 52.
C. You can use should after a number of adjectives, especially:
strange odd funny typical natural interesting surprised surprising
* It's strange that he should be late. He's usually on time.
* I was surprised that she should say such a thing.
D. If ... should...
You can say 'If something should happen...'. For example:
* If Tom should phone while I'm out, tell him I'll phone him back later.
'If Tom should phone' is similar to 'If Tom phones'. With should, the speaker feels that the possibility is
smaller. Another example:
* I've left the washing outside. If it should rain, can you bring it in?
You can also put should at the beginning of these sentences (Should something happen ...):
* Should Tom phone, can you tell him I'll phone him back later?
E. You can use I should .../I shouldn't ... to give somebody advice. For example:
* 'Shall I leave now?' 'No, I should wait a bit longer.'
Here, 'I should wait'= 'I would wait if I were you, I advise you to wait'. Two more examples:
* It's very cold this morning. I should wear a coat when you go out.
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