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.
We often use verbs with modal meanings to talk about permission and obligation.
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?
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 is the most formal way to ask for and give permission.
May I see your passport, please?
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We use can't and mustn't to show that something is prohibited – it is not allowed.
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.
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.
We use have to and must to express obligation. There is a slight difference between the way we use them.
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 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.
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
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.
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.
The LearnEnglish Team
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...." ?
 You must have earned (=must already earn) an undergraduate degree before you can start a master's program.
 You must have finished (=must already finish) cooking when I get home.
I'm looking forward to your explanation. Thank you.
Those sentences are fine. The construction is not actually present perfect, but rather a perfect infinitive. The infinitive has many forms:
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.
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.
The LearnEnglish Team
Thank you , Peter!
Could you please tell me whether after the verb 'need' used as a modal verb may I put the noun? For example, may I say: She need a break or He need resting? Or am I supposed to use only infinitive after 'need' used as a modal?
Thank you in advance.
No, if "need" is a modal verb then it must be followed by another verb. It's usually used in negatives or questions, e.g. "Need she work until late?" or "He needn't work until late".
I hope that helps.
The LearnEnglish Team
Hello team. Could you please help me? What is the mistake in the following sentence? I think it's correct, right?
- You mustn't water the garden; it's still damp.
Some colleagues say that "mustn't" must be replaced with "needn't".