Look at these examples to see how can, can't, must, mustn't, have to and don’t have to are used.
You can put your shoes and coat over there.
You can't leave your bike there.
I must call the electrician and get that light fixed.
You mustn't worry about me. I'll be fine.
You have to have a licence to drive a car.
You don't have to have a licence to cycle on the roads.
Try this exercise to test your grammar.
- Grammar test 1
Read the explanation to learn more.
Grammar explanation
We often use verbs with modal meanings to talk about permission and obligation.
Permission
can
We often use can to ask for and give permission.
Can I sit here?
You can use my car if you like.
Can I make a suggestion?
could
We also use could to ask for permission (but not to give it). Could is more formal and polite than can.
Could I ask you something?
Could I interrupt?
Could I borrow your pen for a moment, please?
may
May is the most formal way to ask for and give permission.
May I see your passport, please?
Customers may request a refund within a period of 30 days.
These pages may be photocopied for classroom use.
Prohibition
We use can't and mustn't to show that something is prohibited – it is not allowed.
can't
We use can't to talk about something that is against the rules, particularly when we didn't make the rules.
What does this sign say? Oh, we can't park here.
You can't take photos in the museum. They're really strict about it.
Sorry, we can't sell knives to under-18s.
must not/mustn't
We use must not to talk about what is not permitted. It is common on public signs and notices informing people of rules and laws.
Visitors must not park in the staff car park.
Baggage must not be left unattended.
Guests must not make noise after 10 p.m.
We use mustn't particularly when the prohibition comes from the speaker.
(Parent to child) You mustn't say things like that to your sister.
(Teacher to student) You mustn't be late to class.
I mustn't let that happen again.
Obligation
We use have to and must to express obligation. There is a slight difference between the way we use them.
have to
Have to shows us that the obligation comes from outside the speaker.
We have to wear a uniform when we're working in reception.
(Student to teacher) When do we have to hand in our homework?
Al has to work tomorrow so he can't come.
We sometimes call this 'external obligation'.
must
Must expresses a strong obligation or necessity. It often shows us that the obligation comes from the speaker (or the authority that wrote the sentence).
I must phone my dad. It's his birthday today.
(Teacher to student) You must hand in your homework on Tuesday or you will lose ten per cent of your mark.
(Sign on a plane) Seat belts must be worn by all passengers.
Note that we don't use must to express obligation in the past. We use have to instead.
I had to pay £85 to renew my passport last week.
No obligation
don't have to
We use don’t have to to show that there is no obligation. You can do something if you want to but it's not compulsory.
You don't have to wear a tie in our office but some people like to dress more formally.
You don't have to go to the bank to do a transfer. You can do it online.
You don't have to come with me, honestly. I'll be fine!
Do this exercise to test your grammar again.
- Grammar test 2
Hi Sokhom,
Yes, I agree that both are grammatically possible in this sentence. However, I guess that have to is more likely to be used. It seems like the speaker is describing the Office's policy or procedure.
If the answer is might (indicating something that's possible but uncertain - i.e., there's a reasonable chance that it will not happen), we must wonder: what will happen if the Office doesn't tell him/her before May? Will he/she be unable to buy a ticket? In a real situation, the speaker would probably go on to comment a bit more about that.
I hope that helps :)
Jonathan
The LearnEnglish Team
Hi LitteBlueGreat,
No worries :)
Yes, it could be must for deduction. That would mean that, based on some kind of evidence, I have arrived at a conclusion that they have no criminal record (without knowing this 100% for certain).
It could also be must for obligation - or in this case, the negative form, must not for prohibition. For example, it is not allowed for people who have been found guilty of an offense to hold certain jobs, and the sentence may be describing this prohibition. The structure is must (not) + verb, and in this case the verb is a perfect passive (have been) since it refers to an already-completed action of 'being found guilty'.
To know which of these meanings is intended, we would need to look at the rest of the text where this sentence is used.
Note that the article should be before the noun phrase: ... found guilty of a criminal offense.
I hope that helps.
Jonathan
The LearnEnglish Team
Hello Danish ahmedd,
Yes, 'could have' + past participle and 'may/might have' + past participle can be used to make guesses about the past.
I'm afraid I can't give a simple answer to your second question. All modal verbs have multiple uses. Some of these uses overlap, and so without knowing exactly what you want to say and the situation you intend to say something, I can't really answer your question. What I can say is that 'must have done' and 'should have done' have different meanings. 'must have done', for example, is often used to make deductions about the past, and 'should have done' can be used to speak about something that didn't happen (e.g. 'He should have gone to school' implies that he didn't go to school).
Hope this helps. You're welcome to ask further questions, but please give us specific examples with an explanation of the situation.
All the best,
Kirk
The LearnEnglish Team
Hello Stellaaa,
We usually speak about 'should' as a way of giving a suggestion rather than communicating an obligation (which is stronger than a suggestion), but it's true that both are similar in that we're telling someone what we think they should do.
All the best,
Kirk
The LearnEnglish Team
Hi Hamdy Ali,
Yes! If you want to recommend this short story, both options work. But, should is clearer, as its primary meaning is to show what you think is the right or correct thing to do. (The primary meanings of can are to show ability or possibility.)
Best wishes,
Jonathan
The LearnEnglish Team