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⚡️Further reading on the differences between of must, mustn't and needn't⚡️
1. The modals must, must not and need not have the same form regardless the subject. There is no ending with he/she/it.
► If you want to say that something is necessary or obligatory, use "must":
I must see a doctor now.
► If you want to say that something is less necessary, use "need":
- I need to wash my car soon.
🔸🔸must vs. mustn't🔸🔸
🔸 You must do something = it is necessary that you do it:
- Don’t tell anybody what I said. You must keep it a secret.
- We don’t have much time. We must hurry.
🔸 You mustn’t do something = don’t do it:
- You must keep it a secret. You mustn’t tell anyone. (= don’t tell anyone)
- We must be very quiet. We mustn’t make any noise.
🔸🔸needn’t and don’t need to🔸🔸
You needn’t do something = it’s not necessary to do it (but you can if you want):
- We have plenty of time. We needn’t hurry. (= it is not necessary to hurry)
- Joe can stay here. He needn’t come with us. (= it is not necessary for him to come)
You can also use don’t/doesn’t need to:
- We don’t need to hurry.
Note that we say ‘don’t need to do’, but ‘needn’t do’ (without to).
► However, if you want to say that something is unnecessary, use need not, not must not. (The negation of must means not allowed to.)
- I must play football. = I have to play football.
- I need not play football. = I do not need to play football. = I do not have to play football.
- I must not play football. = I am not allowed to play football.
Compare needn’t and mustn’t:
- You needn’t tell Steve. I can tell him myself. (= it is not necessary)
- You mustn’t tell Steve. I don’t want him to know. (= don’t tell him)
🔸🔸 needn’t have (done) and didn’t need to (do) 🔸🔸
He needn’t have done something = he did it, but now we know that it was not necessary:
- Why did he get up at 5 o’clock? He needn’t have got up so early. He could have stayed in bed longer.
He didn’t need to do something = it was not necessary to do it. It doesn’t matter whether he did it or not:
- He didn’t need to get up early, so he didn’t.
- He didn’t need to get up early, but it was a beautiful morning, so he did.
You can also say ‘He didn’t have to get up’ in these examples.
Although they both express an absence of necessity in the past, "didn’t need to" and "needn’t have" can have very different meanings.
When referring to an action that happened in the past that was unnecessary, it is more common in British English to use "needn’t have done". Using didn’t need to in this way is also OK.
- You needn’t have brought dessert. (This form is more common in the UK.)
- You didn’t need to bring dessert.
However, when referring to an action that didn’t happen in the past, we can only use "didn’t need to".
- He started to feel better, so I didn’t need to call the doctor. ✅
- He started to feel better, so I needn’t have called the doctor. ❌
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BY English Grammar in Use
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