Unit 35: NECESSITY & ADVICE

Which do you use?
must, should (Unit 33)
have to, need to, needn't (Unit 34)
would (Unit 32)

Present and future time -- affirmative

If I were you, I would see a dentist.
I think you should see a dentist.
I think it's a good idea (this is my advice).
You really must see a dentist quickly.
It's very important
I have to see the dentist tomorrow.
(a fact; it is arranged).

He should get a new car. (I think it is necessary.)
He needs to get a new car. (I know it is necessary.)
He has to get a new car. (He knows it is necessary.)


Present and future time -- negative

You don't need to do this exercise; you are good enough already.
(I think it is not necessary.)
You don't have to do this exercise if you don't want to.
(I am giving you permission not to do it.)
You needn't do this exercise if you don't want to.
(I am giving you permission not to do it.)

You shouldn't do this exercise. (I think it's a bad idea.)
You mustn't do this exercise. (I command you not to do it.)


Past time

We had to go out. (It was necessary, so we went out.)
We should have gone out. (It was necessary, but we didn't go out.)
We didn't need to go out. (It was not necessary.)
We didn't have to go out. (It was not necessary, so we didn't.)
We needn't have gone out. (It was not necessary, but we went out.)
We shouldn't have gone out. (We went out; this was bad.)

The verbs answer two questions at the same time: Was it necessary? and Did it happen?

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Exercise 35.1
Exercise 35.2