We use modal auxiliary verbs and phrases to convey the attitude of the speaker to an event.
Obligation and necessity
We use must/have to to talk about what is right or necessary.
• To express a strong obligation coming from the speaker, we use must or have got to:
I must/have got to do my homework.
• To express a strong obligation coming from an external authority, we use have to or be required to: You have to/are required to obey the law.
• To express a negative obligation or prohibition, we use mustn’t or can’t: You mustn’t/can’t cheat in an exam. (it’s wrong)
• To express lack of obligation or necessity, we use needn’t, don’t need to or don’t have to: You needn’t/don’t need to/don’t have to write this essay. (but you can)
• didn’t need to or didn’t have to mean it was not necessary to do something. The action may or may not have taken place. I didn’t need/have to learn the poem by heart. (so I didn’t)
• Needn’t have means someone did something and later discovered it was not necessary. It indicates regret or relief. I needn’t have learnt the poem by heart. (but I didn’t know that, so I did)
Related verbs
We use verbs like be required to / be obliged to in all tenses to refer to an ‘outside authority’ that gives orders or sets rules:
He was required to get proper qualifications.
We are obliged to come to school on time.
Recommendation and advice
• To express an advisable, but not obligatory action, we use should, ought to or had better.
You should/ought to/had better tell your friend the truth.
For the past, we use should and ought to, but not had better. They suggest the action was not fulfilled.
You ought not to have painted the wall blue. (but you did)
• We can use might or could for an advisable yet probably unfulfilled action, but we have to stress the main verb, not the modal verb.
She might/could say something. (but she probably won’t)
You might/could have told me. (but you didn’t)
Related verbs
We use be supposed to to talk about what should/shouldn’t happen according to rules or according to what is generally expected. It is not used in continuous tenses:
They are supposed to be free.
Animals are not supposed to suffer like this.
Ability and willingness
• To express ability, we use can and be (un)able to: I can’t/am unable to give you my phone.
• To express a general ability over a period of time in the past, we use could or be able to: He could/was able to climb trees when he was younger.
• To express the ability to perform a specificaction in the past, we use be able to or managed to, but not could: She could/was able to/managed to prepare the speech in one hour.
We can use couldn’t in negative sentences:
They couldn’t/weren’t able to/didn’t manage to reach an agreement.
• To express a conditional ability, we use could, but not may or might:
I could/may/might show you how to do it. (but I’m not going to)
• To express our willingness to do something, we use will and would. The meaning is similar to ‘want to’. We use won’t/wouldn’t to express unwillingness.
Nader will always help you if you need something. He is very kind. Her friend wouldn’t even text her.
Related verbs
The police managed to find the criminal.
He succeeded in persuading her to help.
Deduction and possibility
• To express how much we believe something from logical deduction we use must, can’t or might.
He must be an actor. (from the way he talks)
She can’t have studied in Sweden. (she doesn’t even know how to say ‘school’ in Swedish)
It might have been a gift. (I suppose she didn’t buy it for herself, she doesn’t like this colour)
• To say something is possible, we use could, may or might, but not can. He can/could/may/ might be working on a new novel now.
Adding (very) well shows you think the possibility is stronger.
He could/may/might very well have been somewhere else.
Adding even or just shows you think the possibility is weaker.
They could/may/might even have lost all their money. It could/may/might just be true.
• To express a negative possibility, we use may not or might not, but not couldn’t: He couldn’t/may not/might not remember where he left the books.
• To show that we have doubts about something, we stress the pronunciation of the modal verbs:
She may/might/could write something. (but she probably won’t)
He may/might/could have done it. (it’s possible, I can’t remember)
Related verbs
This film is likely to (might) win a number of awards.
It’s a very difficult test. Some people are bound to (are sure to) fail it.
No obligation We use don’t have to and needn’t to mean ‘it isn’t necessary’:
He doesn’t have to be experienced. (but it is not a problem if he is)
Needn’t is a modal verb and has the same meaning as don’t need to:
Pupils don’t need to/needn’t do this.
The past forms of these verbs have differences in their meanings:
She didn’t need to go there. (we don’t know if she went or not)
She needn’t have gone there. (she went, now we know she could have stayed away)
Related verbs Human divers are not required to do the job.
Secret agents are not obliged to be good at maths.
Permission
We use can/could to ask for and give permission. Could is generally a little more polite, though intonation is most important in terms of politeness:
Can/Could I ask you a question?
You can stay out late tonight. / I couldn’t stay out late when I was younger.
Related verbs
Why has water pollution been allowed in this country?
We are permitted to enter the lab.
Prohibition
We use mustn’t, can’t and couldn’t to say that something is not permitted:
People can’t treat animals badly.
Related verbs
Students are not allowed to take mobile phones into exams.
In my last job, we were forbidden to use the Internet in the office.
