Page 359 - Practical English Usage 3ed - Michael Swan, Oxford

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need and used to
Need (see 366) and used to (see 604) are sometimes used in similar ways to modal verbs.
You needn't wait for me.
She used not to be so bad-tempered.
modal auxiliary verbs: meanings two kinds of meaning
Most of the meanings of modal verbs can be divided into two groups. One is to do with degrees of certainty: modal verbs can be used to say for instance that a situation is certain, probable, possible or impossible. The other is to do with obligation, freedom to act and similar ideas: modal verbs can be used to say that somebody is obliged to do something, that he/she is able to do something, that there is nothing to stop something happening, that it would be better if something happened (or did not), or that something is permitted or forbidden.
degrees of certainty
Modal verbs can express various degrees of certainty about facts, situations or events.
a complete certainty (positive or negative): shall, will, must, can't
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b probability (deduction; saying that something is logical or normal):
should, ought to
She should I ought to be here soon.
It shouldn't I oughtn't to be difficult to get there.
c possibility (talking about the chances that something is true or will happen): may
The water may not be warm enough to swim. We may be buying a new house.
d weak possibility: might, could
I might see you again - who knows? Things might not be as bad as they seem. We could all be millionaires one day.
obligation and freedom to act
Modal verbs can express various aspects of obligation and freedom. (These uses of modal verbs are very important in the polite expression of requests, suggestions, invitations and instructions.)
a strong obligation: must, will. need
Students must register in the first week ofterm. All sales staff will arrive for work by 8.40 a.m. Need I get a visa for Hungary?
I shall be away tomorrow.
I shan't be late on Tuesday.
It won't rain this evening.
That can't be John - he's in Dublin.
modal auxiliary verbs: meanings 354
There's the phone. That'll be Tony. Things will be all right.
You must be tired.
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