Cách sử dụng must, mustn't và needn't

CÁCH SỬ DỤNG MUST, MUSTN'T VÀ NEEDN'T 
A. Must - Mustn't - Needn'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 haven't got much time. We must hurry. 
‘You mustn't do something’ = it is necessary that you do not do it (so don't do it): 
- You must keep it a secret. You mustn't tell anybody else. (= don't tell anybody 
else) 
- It's essential that nobody hears us. We mustn't make any noise. 
‘You needn't do something’ = it is not necessary that you do it, you don't need to 
do it: 
- You can come with me if you like but you needn't come if you don't want to. (= 
it is not necessary for you to come) 
- We've got plenty of time. We needn't hurry. (= it is not necessary to hurry) 
B. Thay vì needn't, bạn có thể sử dụng don't/doesn't need to. Vì vậy bạn có thể 
nói: 
 - We needn't hurry, hoặc We don't need to hurry. hoặc We don't need to hurry. 
Nhớ rằng chúng ta nói ‘don't need to do’, nhưng ‘needn't do’ (không có to). 
Needn't và don't need to giống với don't have to (Xem chi tiết TẠI ĐÂY): 
- We've got plenty of time. We don't have to hurry. 
C. Needn't have (done) 
Nghiên cứu tình huống sau: 
George phải đi ra ngoài. Anh ấy nghĩa trời sắp mưa, vì vậy anh ta quyết định mang 
theo ô. 
 Nhưng trời không mưa, vì vậy chiếc ô mang theo trở nên không cần thiết. Vì thế: 
He needn't have taken the umbrella. 
‘He needn't have taken the umbrella’ = He took the umbrella but this was not 
necessary. Of course, he didn't know this when he went out. 
So sánh needn't (do) và needn't have (done): 
- That shirt isn't dirty. You needn't wash it. 
- Why did you wash that shirt? It wasn't dirty. You needn't have washed it. 
D. Didn't need to (do) và needn't have (done) 
I didn't need to = it was not necessary for me to (and I knew this at the time): 
- I didn't need to get up early, so I didn't. 
- I didn't need to get up early, but it was a lovely morning, so I did. 
‘I needn't have (done) something’ = I did something but now I know that it was 
not necessary: 
Tôi dậy rất sớm bởi vì tôi phải chuẩn bị cho việc đi ra ngoài. Nhưng thực tế tôi 
không mất nhiều thời gian để chuẩn bị. So, I needn't have got up so early. I could 
have stayed in bed longer