Can, May, Have To, Must, Should

CAN

We use 'can' to talk about 'possibility'.

• Can you do that?
• I can't manage to do that.
• You can leave your car in that parking space.
• You cannot smoke in here.

Notice that there are two negative forms: 'can't' and 'cannot'. These mean exactly the same thing. When we are speaking, we usually say 'can't'.

We use 'can' to talk about 'ability'.

• I can speak French.
• I can't drive.

We use 'can' to ask for and give permission. (We also use 'may' for this but is more formal and much less common.)

• Can I speak to you or are you too busy?
• You can use my phone.
• You can't come in.

We use 'can' in offers, requests and instructions.

• Can I help?
• Can you give me a hand?
• When you finish that, you can take out the garbage.

We use 'can' with 'see' 'hear' 'feel' 'smell' 'taste' to talk about something which is happening now. (Where you would use the present continuous with most other verbs.)

• I can smell something burning.
• Can you hear that noise?
• I can't see anything.

We can use 'can't' for deduction. The opposite of 'can't' in this context is 'must'.

• You can't be hungry. You've just eaten.
• You must be hungry. You haven't eaten anything all day.
• He was in London one hour ago when I spoke to him. He can't be here yet.

MAY

We can use 'may' to ask for permission. However this is rather formal and not used very often in modern spoken English

• May I borrow your pen?
• May we think about it?
• May I go now?

We use 'may' to suggest something is possible

• It may rain later today.
• I may not have time to do it today.
• Pete may come with us

MIGHT

We use 'might' to suggest a small possibility of something. Often we read that 'might' suggests a smaller possibility that 'may', there is in fact little difference and 'might is more usual than 'may' in spoken English.

• She might be at home by now but it's not sure at all.
• It might rain this afternoon.
• I might not have time to go to the shops for you.
• I might not go.

For the past, we use 'might have'.

• He might have tried to call while I was out.
• I might have dropped it in the street.

MUST OR HAVE TO

We can use 'must' to show that we are certain something is true. We are making a logical deduction based upon some clear evidence or reason.

• There's no heating on. You must be freezing.
• You must be worried that she is so late coming home.
• I can't remember what I did with it. I must be getting old.
• It must be nice to live in Florida.

We also use 'must' to express a strong obligation. When we use 'must' this usually means that some personal circumstance makes the obligation necessary (and the speaker almost certainly agrees with the obligation.)

• I must go to bed earlier.
• They must do something about it.
• You must come and see us some time.
• I must say, I don't think you were very nice to him.

We can also use 'have to' to express a strong obligation. When we use 'have to' this usually means that some external circumstance makes the obligation necessary.

• I have to arrive at work at 9 sharp. My boss is very strict.
• We have to give him our answer today or lose out on the contract.
• You have to pass your exams or the university will not accept you.
• I have to send a report to Head Office every week.

In British English, we often use 'have got to' to mean the same as 'have to'.

• I've got to take this book back to the library or I'll get a fine.
• We've got to finish now as somebody else needs this room.

We can also use ' will have to' to talk about strong obligations. Like 'must' this usually means that that some personal circumstance makes the obligation necessary. (Remember that 'will' is often used to show 'willingness'.)

• I'll have to speak to him.
• We'll have to have lunch and catch up on all the gossip.
• They'll have to do something about it.
• I'll have to get back to you on that.

As you can see, the differences between the present forms are sometimes very small and very subtle. However, there is a huge difference in the negative forms.

• We use 'mustn't' to express strong obligations NOT to do something.
• We mustn't talk about it. It's confidential.
• I mustn't eat chocolate. It's bad for me.
• You mustn't phone me at work. We aren't allowed personal calls.
• They mustn't see us talking or they'll suspect something.

We use 'don't have to' (or 'haven't got to' in British English) to state that there is NO obligation or necessity.

• We don't have to get there on time. The boss is away today.
• I don't have to listen to this. I'm leaving.
• You don't have to come if you don't want to.
• He doesn't have to sign anything if he doesn't want to at this stage.
• I haven't got to go. Only if I want to

SHOULD

We use 'should' for giving advice.

• You should speak to him about it.
• He should see a doctor.
• We should ask a lawyer.

We use 'should' to give an opinion or a recommendation.

• He should resign now.
• We should invest more in Asia.
• They should do something about this terrible train service.

'Should' expresses a personal opinion and is much weaker and more personal than 'must' or 'have to'. It is often introduced by ' I think'.

• I think they should replace him.
• I don't think they should keep the contract.
• Do you think we should tell her.