must + infinitive (must do / must see etc.):
Use must when you think it is necessary or very important to do something:
- The windows are very dirty. I must clean them.
- It`s a fantastic film. You must see it.
- We must go to the bank today. We haven`t got any money.
must is present or future:
- I must go to the bank now.
- I must go to the bank tomorrow.
For the past ( yesterday / last week etc.), we use had to + infinitive ( had to go / had to do / had to write etc.):
- I had to go to the bank yesterday. (= It was necessary for me to go to the bank.)
- We had to walk home last night. There was no bus.
mustn`t (must not)
I mustn`t do it = it is important not to do it, it is a bad thing to do:
needn`t (need not)
I needn`t do it = it is not necessary to do it, I don`t need to do it:
- I needn`t clean the windows. They aren`t dirty.
- You needn`t go to the bank. I can give you some money.
You can also say don`t need to ... (= needn`t):
- I don`t need to clean the windows.
- You don`t need to go to the bank.