English in Use 2

must, mustn't and needn't

you must do sth.= it is necessary that you do it

you mustn't do sth.=it is necessary that you don't do it

you needn't do sth.=it is not necessary that you do it

instead of needn't do, we can use don't/ doesn't need to do

eg. we needn't hurry./ we don't need hurry

当动作发生在过去时，用needn't have done；发生在现在用needn't do 

此外，needn't have done= I did something but now I know that it was not necessary

could/ might have done= things which were possible but not happened

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2.22

should

should do = it is a good thing to do or the right thing to do, or when something is not right or what we expect

should have done= you didn't do it but it would have been the right thing to do 

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3.4

 had better and It's time...

1.I'd better do sth = it is advisable to do it 

2.had better is similar to should but not exactly the same.

3.It's time to do sth

eg. It's late. It's time to go home.

it's time you did something = you should have done it already or started it 

and it's about/ high time ...makes te criticism stronger