Modals: Obligation, Necessity, and Prohibition

EXPRESSING OBLIGATION, NECESSITY, AND PROHIBITION

  • Using modals (must have, have to, should):
    • to show something is necessary and obligatory.
      • You must have a passport to travel,
      • You have to have a passport to travel.
      • You have got to have a passport to travel.
    • to show something is a good idea.
      • You should bring a camera on holiday.
    • to show something is allowed but not necessary.
      • You don’t have to bring a lot of clothes, it’s only a two day holiday.
    • to show something is not allowed.
      • You must not bring drugs into Indonesia.
  • Making modals (must have, have to, should):
    • Must is a modal and does not change to agree with the subject.
      • I must go now. I have a meeting.
    • Have to and have got to are a little different and change to agree with the subject.
      • I have to go now. I have a meeting.
      • He has to go now. I have a meeting.

DIFFERENCE FOR USE

  • Advice vs. Necessity
    • Should and is not as strong as must, have to, and have got to
    • Should shows that something is a good idea and expresses advice but not necessity
  • Have to vs. Have Got To
    • Have to is more neutral than have got to
    • Have got to is very strong, use this when you want to emphasize that you feel something is very important and very necessary
      • John has got to follow his diet because he has a serious illness.
  • Must Not / Mustn’t
    • When you want to show that something is not permitted or is prohibited.
      • You mustn’t smoke in this room.
    • Must not and mustn’t are used as a strong command in situations where the speaker wants to be obeyed.
      • You must not have any more sweet foods.
  • Must, must to, and have got to vs. do not have to
    • Must, have and have got to show that it is necessary to do something.
      • If you want to pass, you must study.
      • If you want to pass, you have to study.
      • If you want to pass, you have got to study.
    • Do not have to show that it is not necessary to do something.
      • If you want to pass, you don’t have to study.
  • Must not vs. do not have to
    • You must do it and You have to do it have the same meaning.
      • You have to have a passport to travel overseas.
      • You must have a passport to travel overseas.
    • You must not do it and You do not have to do it, do not have the same meaning.
    • You do not have to do it means it is not necessary for you to do it.
      • There are no classes on Saturday, so you don’t have to come to school.
    • You must not do it means it is not allowed.
      • You mustn’t smoke inside a movie theater.

TAKING ABOUT THE PAST

  • Have to and Must
    • Refers to regular habits, so you use the present.
      • He has to run every day, but he doesn’t have to swim.
    • To talk about the past, change have and has to had.
      • He had to run last week, but he didn’t have to swim.
    • There is no past tense form of must when it is used to talk about necessity. When you want to express necessity in the past, use have to.

TALKING ABOUT THE FUTURE

  • Must and Have to
    • to talk about necessity in the future, use will before have to.
      • We will have to repair the sink, it is leaking.
      • We will not have to refuel the car, the tank is full.
    • You can also use must to talk about future necessity or prohibition.
      • We must go to the supermarket tomorrow.
      • You must not go out tomorrow.