must and have to; mustn’t, haven’t got to, don’t have to, don’t need to and needn’t

  • Must and have (got) to are not exactly the same. We usually use must to give or ask for orders — the obligation comes from the person who is speaking or listening.
    We use have (got) to to talk about an obligation that comes from ‘outside’ — perhaps because of a law, or a rule, or an agreement, or because some other person has given orders. Compare:
      I must stop smoking. (I want to.)
      I’ve got to stop smoking. Doctor’s orders.
      This is a terrible party. We really must go home.
      This is a lovely party, but we’ve got to go home because of the babysitter.
      I’ve got bad toothache. I must make an appointment with the dentist.
      I can’t come to work tomorrow morning because I’ve got to see the dentist. (I have an appointment.)
      Must you wear dirty old jeans all the time? (= Is it personally important for you?)
      Do you have to wear a tie at work? (= Is it a rule?)
  • Mustn’t is used to tell people not to do things: it expresses ‘negative obligation’.
    Haven’t got to, don’t have to, don’t need to and needn’t are all used to say that something is unnecessary. They express absence of obligation: no obligation. Compare:
      You mustn’t tell George. (= Don’t tell George.)
      You don’t have to tell Alice. (= You can if you like, butit’snot necessary.)
      You don’t have to wear a tie to work, but you mustn’t wear jeans.
      (= Wear a tie or not, as you like. But no jeans.)
      Haven’t got to, don’t have to, needn’t and don’t need to all mean more or less the same.