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 show 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?
• Ali has to work tomorrow so he can’t come.

We sometimes call this an ‘external obligation’.

must

Must express 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 call my dad. It’s his birthday today.
• (Teacher to student) You must hand in your homework on Tuesday or you will lose ten percent of your mark.
• (Sign on a plane.) Seat belts must be worn by all passengers.

Note that we didn’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 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!