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must mustn't don't need to
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.
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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.