1. necessity (MUST, HAVE TO, BE TO) *I’m afraid I must/have to go. *The lecture is to begin at 8.
2. order, command (MUST, BE TO) *You must stay here till 9. *You are to tell me everything.
3.urgent, request emphatic advice (MUST)
*You must come and have dinner with us some day
4.prohibition (MUST, BE TO) *You must not ask for things. *You are not to say a word to her.
5. strong probability, near certainty (MUST),
supposition bordering on assurance
*She must be about 20.
MUST expresses duty, obligation, necessity from the speaker’s point of view; have – necessity arising out of circumstances, and be denotes an expected action (an action which is necessary because somebody expects it to be done) according to a plan, an arrangement, etc.
BE may denote sth unavoidable, sth bound to happen.
*It was the girl who was to become my mother.
2. In the meaning of necessity MUST, BE are found in affirmative and interrogative sentences and followed only by the indefinite infinitive.
HAVE is found in all kinds of sentences and is combined only with the indefinite infinitive. In negative sentences HAVE denotes absence of necessity.
The past tense of BE with the perfect infinitive denotes an unfulfilled plan.
In the imperative meaning MUST is retained in indirect speech in past-time contexts:
*He told me I mustn’t cry.
Imperative meanings are found in affirmative and negative sentences and followed by the indefinite infinitive.
BE is used to make orders and prohibition stricter.
Imperative sentences may express different degrees of strictness. Compare the following (milder forms are given first) :
*You mustn’t write me letters.
*You are not to write me letters.
*You can’t write me letters.
*You may not write me letters.
5. In the meaning of near certainty, strong probability MUST is found only in affirmative sentences and is followed by different forms of the infinitive.
If reference is made to the present, the continuous infinitive is generally used (with the exception of verbs that are not used in the continuous forms): if MUST is followed by the indefinite infinitive it expresses obligation, necessity:
*She must take better care of her health. (necessity)
*She must be taking very good care of herself. (supposition)
MUST followed by the perfect infinitive refers the action to the past.
MUST cannot express near certainty about a negative or a future action. In such cases we use probably, evidently, or to fail or negative prefixes.
*Probably he’ll speak English.
*He must have failed to recognize you.
*He must have misunderstood me.
7. BE can express possibility (like CAN or MAY) when followed by the passive infinitive:
*Where is the book to be found?
8. Notice the use of set-phrases:
(a) In leave-taking: *I must be going. *I must be off.
(b) To express invitation: *You must come and see us some time.
(c) *I must say/tell you that…, where the meaning of obligation is weakened.
(d) *What am I to do? *What is to become of me? *Where am I to go?