Obligation and Necessity



Obligation and Necessity


Must and Mustn’t


  • You use MUST to give people orders or to make strong suggestions. This is especially common when you are in a position of authority. EG a parent talking to a child.
    • You must be home by ten! (You can’t stay out later than ten.)
    • You must listen to me. (You can’t not listen to me.)


  • You use MUSTN’T to give a negative order to someone or to tell them about a negative order.
    • You mustn’t talk to strangers. (You can’t talk to strangers. It might be dangerous.)
    • You mustn’t park here. (You can’t park here. There’s a ‘No Parking’ sign.)


  • You use MUST and MUSTN’T to talk about an obligation you feel.
    • I must post this letter.
    • We mustn’t be late for the lecture.



Have to, Don’t have to and Needn’t


  • You use HAVE TO to talk about obligations that come from other people. Often the ‘other people’ are the police, the government, the law, teachers etc. In the positive form, HAVE TO is very similar to MUST.
    • You have to drive on the left. (You can’t drive on the right. Te law says you must drive on the right.)


  • You use DON’T HAVE TO and NEEDN’T to say something is unnecessary.
    • I don’t have to get up early tomorrow. (It isn’t necessary)
    • You needn’t get up early tomorrow.


  • DON’T HAVE TO & NEEDN’T are very different in meaning to MUSTN’T.
    • You mustn’t leave. (You can’t leave. You have no choice)
    • You don’t have to leave. (You can stay or go, it’s your choice)



Form of Must, Have to and Needn’t

  • Be careful! MUST doesn’t have a past or future form. You use HAD TO to talk about the past and WILL HAVE TO to talk about the future.
    • He had to start work at six. (not “He musted…)
    • You’ll have to find a job. (not “You’ll must…)


  • You use the infinitive without to after MUST and NEEDN’T
    • You must call me
    • You mustn’t smoke
    • You needn’t worry