Modal verbs to express obligation.

• Have to / Don't have to / Must / Mustn't.
  • Use have to + INFINITIVE to talk about rules and obligations.
    • She has to get up very early.
  • Use don't have to + INFINITIVE to say there is no obligation, or something is not necessary.
    • You don't have to work on 11 June.
  • Use must + INFINITIVE to talk about rules and obligations.
    • You must do your homework.
  • Use mustn't + INFINITIVE to say something is prohibited.
    • You mustn't smoke here.
• Have to / Must.
  • Use must or have to when you say what you think is necessary, when you give your opinion.
    • Its a fantastic film. You must see it or You have to see it.
  • When you are not giving your opinion, use have to (not must).
    • In many countries, men have to do military service. (This is not my opinion, it's the law in those countries.)
• Mustn't / Don't have to.
  • Mustn't and don't have to have completely different meanings.
    • You mustn't go. = It's prohibited.
    • You don't have to go. = You can come if you want, but it's not necessary.
• Exercise: Complete the following sentences with must /mustn't or don't have to.
  1. Gary gave me a letter to post. I __________ forget to post it.
  2. There's plenty of time. You __________ decide now.
  3. This is a valuable book. You __________ lose it.
  4. A) What sort of house do you want to buy? Something big?
     B) Well, it _______ be big - that's not so important, but it ______ have a nice garden.
• Answers.
  1. Gary gave me a letter to post. I mustn't forget to post it.
  2. There's plenty of time. You don't have to decide now.
  3. This is a valuable book. You mustn't lose it.
  4. A) What sort of house do you want to buy? Something big?
     B) Well, it doesn't have to be big - that's not so important, but it must have a nice garden.