Essential Grammar in Use(Third Edition)--Modals, imperative etc. Unit31

must            mustn't          don't need to

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A

must + infinitive (must do / must work etc.):

I/we/you/they he/she/it

must

do go see eat   etc.

B

I must (do something) = I need to do it:

• I'm very hungry. I must eat something.
• It's a fantastic film. You must see it.
• The windows are very dirty. We must clean them.

For the past (yesterday / last week etc.), we use had to ... (not must):

• I was very hungry. I had to eat something. (not I must eat)
• We had to walk home last night. There were no buses. (not We must walk)

C

mustn't (= must not)

I mustn't (do something) = it is necessary not to do it.

it is the wrong thing to do:

• I must hurry. I mustn't be late.
• I mustn't forget to phone Jane. (= I must remember to phone her)
• Be happy! You mustn't be sad. (= don't be sad)
• You mustn't touch the pictures. (= don't touch the pictures)

D

don't need to

I don't need (to do something) = it is not necessary:

• I don't need to go yet. I can stay a little longer.
• You don't need to shout. I can hear you OK.

You can also say don't have to ...:

• I don't have to go yet. I can stay a little longer.

Compare  don need to and mustn't:

• You don't need to go. You can stay here if you want.
• You mustn't go. You must stay here.