The Modal Verb - Must

Must is a modal verb of obligation and necessity. We can also use have to for obligation and necessity.For example :- You must study hard if you want to do well. (Personal obligation)- You have to wear uniform at many schools in England. (Exteranal obligation, a rule or a law)
In the negative, however, the meanings are completely different.

For example :

• You mustn't smoke here (Mustn't means that it is forbidden. it isn't allowed).
• You don't have to come if you don't want to. (Don't have to means that it is not neccesary).

If we want to talk about obligation and necessity in the past or the future, we have to use a suitable form of have to:

For example :

• We have to cancel tommorow's meeting (Future)- We had to cancel yesterday's meeting (Past).
• We have had to cancel all meetings this week (Present Perfect).

Question And Answer

1. In Britain you have to/mustn't wear a seatbelt when you're in a car.

2. You don't have to/mustn't park hear. It is not allowed.

3. You really must/have to talk to Jim. He's so upset.

4. Yester day I've had to/had to wait almast an hour for my bus. I was furius.

5. If you want to catch the early train tomorow morning, you'll have to/musn't get up at 6 am.