In Part 2 of my series on modal verbs, I’d like to review the functions of obligation (necessity) and prohibition. Part 1 dealt with the functions of ability and habits.

In that last post, I said that I would be splitting the series into three posts. Having researched the topic in more depth, I’ve decided that I will need four posts to cover modal verbs the way I would like. In my last post I said that I would be covering the function of permission in this part. However, I shall address that in a later post.

We use must and have to when we say that something is necessary. We can also use need to.

  • We must finish the meeting by 3pm because I have to get to the airport by 5pm.
  • You need to sign this form in two places, here and here.

In writing, must and have to have the same meaning. However, in spoken English there is a small difference.

With have to”, the situation makes something necessary, whilst with “must” the speaker personally feels that something is necessary.


  • I have to pay my credit card bill by the end of the month. (It’s the credit company’s rule)
  • You must take a break. (I’m telling you – it’s my strong advice)
  • I must stop smoking. ( I feel I need to)
  • Children have to attend school until the age of 16. (it’s the law)


Question Form
To make a question we normally use “have to”. If we use “must” in a question, we can sound annoyed or irritated especially if we stress “must” in speech.

  • Do you have to work this evening?
  • Must you work this evening? I have tickets for the theatre. (I’m not happy!)

“Have got to” and “will have to” are also used for necessity. They are more informal.

  • I’ve got to finish this sales report before I leave for my holidays.
  • I will have to phone my clients tomorrow and inform them of the change in plans.

When something is not necessary and you have a choice we use “don’t have to”. We can also use “don’t need to/needn’t”.

  • You don’t have to drive the car this evening. You can catch a taxi. (you have a choice)
  • We don’t have to wear a suit on Fridays. (you have a choice)
  • You don’t need to/needn’t wait up for me tonight. I have a key.


When something is prohibited or forbidden. we use can’t, be not allowed and mustn’t.

  • You can’t park here. (those are the rules)
  • You are not allowed to smoke in the office.
  • You mustn’t enter this room. It’s strictly forbidden.

and have to have the same meaning in positive sentences but different meanings in negative sentences.

  • I have to/I must leave today (it’s necessary)
  • I don’t have to leave yet. (I have a choice)
  • I mustn’t leave now. (it’s important that I don’t)



Obligation (necessity)
We use “had to”. There is NO past form of must.

  • I had to get the sales figures for my boss before the meeting.

No Obligation
We use “didn’t have to

  • You didn’t have to get me a present. That’s very kind of you.

We use “couldn’t” or “wasn’t allowed to”

  • When I was young we weren’t allowed to watch television in the evenings.
  • I arrived at the airport after check-in had closed, so I couldn’t catch my flight.


If you want to give someone advice about what you think is best or most sensible, you use “should”, “should not (shouldn’t), “ought to” and “ought not to (oughtn’t to)

  • You work too much. You should take more breaks. (it’s my advice)
  • You haven’t been feeling well for a while. You should go to the doctor.
  • You shouldn’t leave the bicycle there. It could be stolen.

Note that the above sentences are all “soft” advice that is given.

If you want to give strong advice to someone, you use “must” or “have to”. The strong advice becomes more of a necessity.


  • You are really not well. You must/have to go to the doctor’s. (It’s necessary)
  • You have to/must practise more if you want to pass the piano exam. (It’s necessary)


Click here if you’d like to test yourself with this exercise on modal verbs of obligation. It was prepared by Perfect English Grammar.

In the next post, I will cover modal verbs of probability. In the meantime, please do not hesitate to ask me any questions you may have on this topic.

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Ciao for now.


Business Grammar Builder, Paul Emmerson (Macmillan) 2010
Perfect English Grammar