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 hire a new sales assistant.
I think you should hire a new sales assistant.
I think it's a good idea (this is my advice).
You really must hire a new sales assistant quickly.
It's very important
I have to hire the sales assistant tomorrow.
(a fact; it is arranged).

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

Present and future time -- negative

You don't need to pay for this shirt; you've bought me enough already.
(I think it is not necessary.)
You don't have to pay for this shirt if you don't want to.
(I am giving you permission not to do it.)
You needn't pay for this shirt if you don't want to.
(I am giving you permission not to do it.)

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

Past time

We had to take inventory. (It was necessary, so we took inventory.)
We should have taken inventory. (It was necessary, but we didn't take inventory.)
We didn't need to take inventory. (It was not necessary.)
We didn't have to take inventory. (It was not necessary, so we didn't.)
We needn't have taken inventory. (It was not necessary, but we did it.)
We shouldn't have taken inventory. (We took inventory; 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