A. Can, may, and must are only used in the present tense. If we want to use other tenses, we use different forms. This lesson will teach us how to express the future and the past.
Can: be able to
- Will you be able to walk after the operation?
- I was busy, but I was able to answer his email.
May: be allowed to
- Will you be allowed to join us at the weekend?
- We weren’t allowed to talk during the exam.
Must: have to
- I will/’ll have to water the garden tomorrow.
- I had to finish my report yesterday.
B. The same modal verb can be used to express different things in English.
Can: possibility, criticism, ability, request, opportunity
- She can run very fast.
- Can you help me?
- I’m seeing her tomorrow so I can tell her.
- You can practise your French on holiday in Paris.
- You can be really moody, you know!
Can’t/cannot: negative of can; can/cannot also express rules
- He can’t swim.
- You can’t speak loudly in the library. (a rule)
Could/couldn’t/could not: to talk about what someone was/wasn’t able or allowed to do
- I could swim when I was four.
- The children were so loud that we couldn’t hear each other.
- The shop was closed, so I couldn’t buy anything.
Note 1:
a) could (general ability)
- My grandpa could speak five languages.
b) was/were able to (ability, success – we are talking about what happened in a particular situation)
- He was able to escape. (He managed to escape).
May: permission
- May I borrow your pen?
- May I stay up late?
Must: rules, obligations, strong recommendations
- You must do your homework before you go out.
- She must get up early tomorrow.
Mustn’t/must not: prohibition
- You mustn’t smoke there.
- You mustn’t eat in the gym.
Note 2:
Must or have to?
They have a very similar meaning, so you can usually use either form. However, there are some small differences.
Must indicates our feelings and is used more often for specific obligations, or in formal English.
- I must go to the dentist. (it’s my own decision)
- I must tell you something. (a specific obligation, on one occasion)
- Baggage must not be left unattended. (formal)
Have to is used to express general, external obligations, e.g. rules and laws, something that somebody in authority has said is necessary.
- In our school, we have to wear a uniform. (a school rule)
- I have to brush my teeth twice a day. (experts say so)
Note 3:
Have got to: is used in spoken English, it is informal.
- I’ve got to make dinner now.
- It’s late, I’ve got to go home.
Don’t have to/needn’t: it is not necessary to do something
- It’s free of charge. You don’t have to pay.
- You don’t have to pay me back immediately.
Didn’t have to/did not have to: the action was not necessary and we did not do it
- It was raining yesterday so I didn’t have to water the garden.
- We were on holiday so we didn’t have to get up early.
Should/shouldn’t: we use it to give advice or an opinion about what is the correct or best thing to do and expectations.
- You should see a doctor. (advice)
- You shouldn’t eat so much sweets.
- He should be here by now. (expectation)
Ought to/oughtn’t to: is interchangeable with should/shouldn’t. It is stronger in meaning so be careful not to offend somebody.
- You oughtn’t eat so much sweets.