Modal verbs: obligation, lack of obligation and prohibition

Strong obligation: must, have to, (informal) have got to
must
Must is used to talk about strong obligations in the present and future that are imposed by the speaker. It is also used to talk about laws and rules. To talk about the past we have to use had to.
I must arrange to have a telephone alarm call.
I had to take a taxi.

have to / have got to
Have to/have got to are used to talk about strong obligations in the present and future that are not imposed by the speaker.
I’ve got to work nights this week. (My boss says so.)
If in doubt whether to use must or have to, use have to.

had to
Had to is used to talk about past and reported obligations.
I had to help on the farm when I was young.
We were told we had to get a work permit before we started the job.

Lack of obligation: doesn’t/don’t have to
Don’t have to is used to talk about things that aren’t obligatory – they are optional.
She doesn’t have to come to the meeting if she doesn’t want to.
We didn’t have to wear a uniform at our school.

Prohibition:
mustn’t
Mustn’t is used when something is forbidden.
You mustn’t use machinery when you are sleepy.


These activities practise this Grammar: