Obligation and prohibition

You have to switch off your phone during take-off and landing.

You must be on time tomorrow because there's a test.

obligation

Have to and must are normally used to talk about obligation, or something that it is necessary to do.

Have to and must have a very similar meaning and you can usually use either form. Have to is more common for general external obligations, for example, rules and laws. Must is more common for specific (i.e. on one occasion) or personal obligations. Compare:

I have to wear a shirt and tie at work.

It's the rule in this company

I must buy a new shirt - this one is too old now.

It's my own decision

strong recommendation

We can also use have to or must for strong recommendations.

I love the Louvre! You have to go when you're in Paris.

You must see this film - it's amazing!

Have to is a normal verb and it exists in all tenses and forms, e.g. also as a gerund or infinitive.

Must is a modal verb. It only exists in the present, but it can be used with a future meaning.

I had to wear a uniform at my primary school.

I hate having to get up early. My interview is at 9.00.

Do you have to work on Saturday?

Must I switch my phone off now?

You don't have to pay - this museum is free.

You don't have to go to the party if you don't want to.

You mustn't park here.

You mustn't eat that cake - it's for the party.

We use don't have to when there is no obligation to do something and musn't when something is prohibited.

Don't have to and musn't are completely different. Compare:

You don't have to drive - we can get a train.

• You can drive if you want to, but it isn't necessary / obligatory.

You mustn't drive along this street.

• It's prohibited, against the law.

We can often use can't or not allowed to instead of musn't.

• You mustn't park here.

• You can't park here.

• You're not allowed to park here.

You should take warm clothes with you to Dublin. It might be cold at night.

You shouldn't drink so much coffee. It isn't good for you.

We use should to give advice or an opinion. Should is not as strong as must / have to.

• Should is a modal verb. The only forms are should / shouldn't.

Ought

We can use ought to / ought not to instead of should / shouldn't.

• You ought to take warm clothes with you to Dublin.

• You ought not to drink so much coffee.

We use should to give an opinion - to say if we think something is the right or wrong thing to do.

I think the government should do something about unemployment.

Exercise

You mustn't use your phone in quiet zones.

Do you think we should text Dad to tell him we'll be late.

Do you think we ought to text Dad to tell him we'll be late.

You mustn't send text messages when you are driving.

A pilot has to wear a uniform when he's at work.

A pilot must wear a uniform when he's at work.

You should go to the Uffizi when you're in Florence.

You must go to the Uffizi when you're in Florence.

I have to speak to my phone company. My last bill was wrong.

I must speak to my phone company. My last bill was wrong.

We don't have to hurry. We have plenty of time.

When I was at school we had to wear a horrible uniform.

You shouldn't walk on the grass. They've just planted flowers there.