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?
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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, I have a question about how to use can't must not and musn't. We use can't , When we see any signal or rule and we can't do something, for example you must not park here, so I understand I can't park there because it's prohibited? and in the case with musn't is when I say to someone or to myself I can't or they can't do that?
With Have to or Must. We use have to when someone tell me I have to do something and must is when I think someone or myself must do something. for example: I have to do my homework because my teacher said it's very importan and with must - I must wear that dress because it's very beautiful or You must turn on the tv it's very important to watch the news
Hello Javi,
Yes, one of the uses of 'can't' is to talk about prohibitions or things that are not allowed. If I see a sign that says 'No Parking' and my friend is trying to park there, I can say to her 'You can't park here.' In this case, I'm making an observation about a rule, but I'm not making the rule myself.
It would be unusual for you to say 'You mustn't park here' in this situation because the prohibition isn't coming from you -- you're just pointing out what the rules are. It's not impossible to say it, but it would be a bit strange.
'have to' is much more commonly used than 'must', especially outside the UK. When 'must' is used, it's often a kind of self-imposed obligation and sounds almost like a reminder one gives oneself about something important. It can also be used on signs and sometimes authority figures (for example, teachers) will use it. In these cases, notice that it's the authority figure who is speaking to the person who has to follow the rules. 'have to' is more often used when we the people who have to follow the rules talk about the rules that are imposed on us.
Does that make sense? By the way, we have another set of pages on Modal verbs that you might find useful.
All the best,
Kirk
LearnEnglish team
Hello!
Could you please explain the following:
Could I ask "Have I got to wear the uniform" or "Do I have to wear the uniform?" Is there a difference? And could one answer to this question like
"You should" or "You must" instead of "I have to"?
Thank you so much for your help, and for the answer to this question!!! You help so many people to learn English, thanks!!!
Hello howtosay_,
The forms 'have got to...' and 'have to...' have the same meaning here (obligation). 'Have got to...' is a little more informal.
The answer depends on what you want to say: should is more used to give advice, have to describes a rule or requirement and must generally describes a personal sense of obligation. All are possible grammatically; it depends on the context and what the speaker wants to say.
Peter
The LearnEnglish Team
Hi Peter,
Thanks so much for your prompt reply.
Is the construction of "must (=obligation) + perfect infinitive", like "You must have earned (=must already earn) an undergraduate degree before you can start a master's program" commonly used in spoken English only or also in written English? Is it mostly used in British English or also in American English ?
Because in my grammar books, this construction in the sense of the above implication (must already....) is not available. Furthermore, at one time I wrote to an online American English teacher using this structure, she corrected my sentence by deleting the perfect infinitive to read "You must earn an undergraduate degree.....".
I don't understand why she deleted the perfect infinitive.
I would appreciate it very much if you could help me with your further explanation about this use, so I can understand it better.
Best regards,
Hi again melvinthio,
The construction 'must already have earned' is perfectly fine. However, in some dialects of American English past forms are more common with 'already', so where in Britain we would say 'I have already done it' in the US 'I already did it' is more common. This may be the reason the teacher chose to correct what you wrote.
The present modal is also possible, of course. In this case it is looking forward (describing an event after the moment of speaking), while the perfect modal is retrospective (describing an event before another later event). The choice is up to the speaker.
Peter
The LearnEnglish Team
Hi Peter,
I'd like to ask for your help for my following questions.
Is it grammatically right to use "must (=obligation) + present perfect tense" for present and future references in the sense of "must already...." ?
E.g. :
[1] You must have earned (=must already earn) an undergraduate degree before you can start a master's program.
[2] You must have finished (=must already finish) cooking when I get home.
I'm looking forward to your explanation. Thank you.
Best regards,
Hi melvinthio,
Those sentences are fine. The construction is not actually present perfect, but rather a perfect infinitive. The infinitive has many forms:
and even
Of course, the last of these is very rare and quite questionable stylistically; we would usually avoid it by using an active form, for example. However, grammatically it is possible.
Peter
The LearnEnglish Team
Hi Teacher ,
"I'll __have to___ wear the black dress tonight. The invitation said formal clothes."
Can we also use "I must wear......." ?
Is there any difference between them?
Hello The Best of The World,
Both must and have to express obligation. The main difference between them is where the obligation comes from. With must we generally see the obligation as coming from the person (their morality, their beliefs etc), while with have to we see the obligation coming from elsewhere (rules, regulations etc). That said, the difference is very subjective and the two are very often interchangeable. In your example, I think both options sound fine.
Peter
The LearnEnglish Team