Modal Verbs of Obligation & Prohibition


In the positive, we have 2 forms to show obligation, have to and must.

The difference is that have to/has to is an obligation that comes from other people, but mustcomes from ourselves. We use have toto show that the obligation comes from outside of ourselves. It is possible to use need to instead of have to.

  1. a)Ihave to do my homework once a week.
  2. b)You need to do two tests every week.
  3. c)I must study as much as I can.

Both show that there is an obligation to do or be something.

In the negative, must not (mustn’t) and don’t/doesn’t have tohave different meanings. Mustn’t means that we are not allowed to do something. It is prohibited. Don’t have to, however, means that you have a choice. You can if you want to, but it is not a necessity.

  1. d)She doesn’t have to go to university if she doesn’t want to.
  2. e)I mustn’t forget to take my medication.



There is no past of must. Instead, in a positive sentence we use had to. In the negative, we use was/were not allowed to.

For have to, use didn’t have to (meaning we had a choice) and had to, where there was an obligation.

  1. f)I had to go to school at 7am every morning.
  2. g)I wasn’t allowed to be late for school.
  3. h)We didn’t have to do sport after school, but most of us wanted to.



We use have to for both must and have to/has to.

Form: sb + will + have to + bare infinitive + ob.

  1. i)We will have to take three classes per term in my final year of university.
  2. j)You will have to work later next month.




Present Positive:

Must + sb + bare infinitive + ob…?

  1. a)Must I take this medication?

Do/Does +sb + have to + bare infinitive + ob…?

  1. b)Does she have to go to the doctor every week?

Present Negative:

Mustn’t + sb + bare infinitive + ob…?

  1. a)Mustn’t I see my lecturer today?

Don’t/Doesn’t + sb + bare infinitive + ob…?

  1. b)Don’t we have to check in before midday?

Past Positive:

Was/were + sb + allowed to + bare infinitive + ob…?

  1. c)Were we allowed to smoke at school? I can’t remember.

Did + sb + have to + bare infinitive + ob…?

  1. d)Did we have to take our ID cards to school?

Past Negative:

Wasn’t/weren’t + sb + allowed to + bare infinitive + ob…?

  1. e)Weren’t we allowed to leave school at 16?

Didn’t + sb + have to + bare infinitive + ob…

  1. f)Didn’t we have to take exams at 16?


Future Positive:

Will + sb + have to + bare infinitive +ob…?

  1. g)Willwe have to learn a new language to get our degree?


Future Negative:

Won’t + sb + have to + bare infinitive + ob?

  1. h)Won’t we have to write a thesis of 100 000 words?


Exercise 1.- Grammar


Complete the exercise with a suitable modal verb of obligation or prohibition.


  1. a)I ________ remember to return these books to the library, or I will get a fine.


  1. b)We _____________wear a school uniform when we were at school.


  1. c)______ you __________ take exams twice a year when you were at school?


  1. d)I ________drink as I’m driving tonight.If I do, I could cause a terrible accident.


  1. e)You ______________drive home. You can take a taxi.


  1. f)We __________________drink alcohol on school property, even at parties.


  1. g)We_____________________ smoke if we were wearing our school uniform.


  1. h)They______________ play sport every day after school as it was part of their curriculum.

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