Expressing obligation and permission
Obligation
must and have to
We can often use must and have to without any difference in meaning:
Surgeons have to / must carry out their operations efficiently and safely to ensure their patients make a swift recovery.
We use must + infinitive without to in the present tense. For other tenses, we use have to + infinitive:
I'd love to go on an adventure holiday, but I'll have to save up enough money first.
In order to get a tourist visa for the USA, I had to fill in an online application form and make an online payment.
We use have to more often than must in questions:
Do we have to complete every section in the survey?
must
We use must for a goal or an obligation that we give ourselves:
I must make a bigger effort to meet the targets I set myself.
We use must for strong advice:
You must be extremely attentive when you start learning how to handle birds of prey.
have to
When the obligation comes from someone else, we user have to, and not must.
My manager has just handed me a list of jobs that I have to complete by Friday at the latest.
supposed to
We use be supposed to + infinitive to talk about an obligation which is different from what really happens:
We're supposed to do circuit training for two hours a day. (But most people do much less.)
Aren't you supposed to be doing group work now? (i.e. not chatting with friends over coffee)
should
We use should + infinitive without to to talk about the right thing to do, but which is different from what really happens:
You should always use your own words when you write an essay, not copy texts you have found on the internet.
The past of should is should have + past participle:
You shouldn't have answered that question in only 150 words!
ought to
Ought to means should.
In some cultures, young people are taught that they ought to be especially respectful towards older people.
had better
We use had better to give strong advice or to tell someone (including ourselves) what to do.
I‘d better go home. It’s getting late.
You‘d better tell Mum what happened before she finds out.
Had better often implies that something negative will happen if something is not done, and it often sounds like a threat or a warning.
He‘d better give me my money back, or he’ll be in trouble.
You‘d better behave yourself if you don’t want me to get really angry.
Permission
can, let, allowed to and may
To express permission, we use can (past could), let, allowed to and may (past was/were allowed to):
You can drive to college, but you must park your vehicles in the spaces allocated to students.
Am I allowed to use my student identity card to get reduced fares on public transport?
She let him copy her notes before the exam.
We use may in formal situations:
You may go and wait outside the interview room when you have completed the questionnaire.
don't have to, don't need to and needn't
To say that there is no obligation, or it's not necessary, we use don't have to, don't need to and needn't:
You don't have to take your shoes off, if you don't want to, but it is expected when you are invited into most homes in Sweden.
You needn't wear a suit and tie for this job, but it's important to dress smartly.
didn't need to
I didn't need to means 'It wasn't necessary and I didn't do it'
I didn't need to read the novel to find out about the plot of the story because I'd already seen the film.
needn't have
I needn't have means 'It wasn't necessary but I did it':
You needn't have bothered to bring a present, but thanks very much anyway. It's much appreciated.