Must is used mostly to talk about certainty, necessity and obligation. It only have a present simple form so we have to use other modals to talk about certainty, necessity and obligation in the past or future.

  • Certainty

Must is used to say that something is certain or highly probable

She must have a problem – she keeps crying.

To talk about something being certain or highly probable but in negative way we use cannot/can’t:

She can’t be very happy – she keeps crying.

  • Necessity and obligation

We can use must or have to to say what is necessary and give strong advice and orders. The difference between them is that with must the speaker is giving the order or agrees with it, but with have to the speaker is talking about an instruction given to them.

You must finish your work before you leave. (Boss speaking)

I must finish my work before I leave. (I want to finish the work)

I have to finish my work before I leave. ( I am ordered to finish)

To say that something is not necessary we use need not (needn’t) or don’t have to

You need not work on Saturday, if you don’t want to. (means you can work if you want, the choice is yours) 

You don’t have to work on Saturday, if you don’t want to. (means you can work if you want, the choice is yours) 

We use must not (mustn’t) to say that it is wrong to do something, or to tell people not to do things. You can also use not be allowed to in this case

You mustn’t use my car.

You are not allowed to use my car

  • Past Necessity and obligation

Must has no past form so to talk past necessity and obligation we use had to

When I was a child I had to walk to school.

  • Future Necessity and obligation

Will have to is used to talk about necessity and obligation in the future

I will have to get a job when I finish university.

1. must/have to/need to

UseExamples
certainty (positive)You must be happy with your exam results.
certainty (negative)You can’t be happy with your exam results.
necessityI must go to work on Saturday.
necessityI have to go to work on Saturday.
necessity – absense ofI needn’t go to work on Saturday if I don’t want to.
necessity – absense ofI don’t have to go to work on Saturday if I don’t want to.
possibilityYou must be tired.
advice, recommendationYou must see the new James Bond film..

2. must not

UseExamples
prohibition You mustn’t use my car.
You are not allowed to use my car.