Практическая грамматика

§ 4. Must.
The verb must has only one form. The expressions to have to and to be obliged to which have the same meaning can be used to supply the missing tense forms of the verb must. Must is used in present- time contexts with reference to the present and future and in combination with the Perfect Infinitive it refers to the past. In past- time contexts this form is used only in reported speech, i. e. the rules of the sequence of tenses are not observed with must.
And now I must go back to my social duties. (Voynich)
Must expresses obligation, necessity, an urgent command or prohibition, and a supposition bordering on assurance.
1. Obligation, necessity, duty (from the speakers point of view).
(a) due to circumstances (in this meaning it is equivalent to have to and is used only with the Indefinite Infinitive in affirmative, negative and interrogative sentences)
He must write. He must earn money. (London)
I must ask you to excuse me. (Maugham)
Then we must find another way. (Voynich)
The absence of necessity is expressed by need not.
Must I go there to-morrow? Yes, you must. No, you needn't.
(b) arising out of the nature of man and consequently inevitable.
All experience tended to show that man must die. (Galsworthy)
Bad seed must produce bad corn.
2. A command, an urgent (emphatic) request or a prohibition. In this meaning it is used only with the Indefinite Infinitive.
You must leave the room at once!
You mustn’t walk on the grass!
You must come and see us when you are in London.
3. Probability or supposition bordering on assurance.
In this meaning must is used with all the forms of the Infinitive in affirmative sentences only. It corresponds to the Russian должно быть.
If the action refers to the present the Indefinite Infinitive is used; if the action refers to the past the Perfect Infinitive is used.
Surely, they don't want me for myself. Then they must want me for something else. (London)
John must be at the station by now.
Oh, Mae, think how he must be suffering! (Webster)
That must have been a pleasant experience (Voinich)
It must have been raining all night. There are big puddles
in the garden.
In negative sentences supposition is expressed by means of the modal word evidently.
Evidently, she did not know my address.
Supposition referring to the future cannot be expressed by must. The modal word probably or the modal expression to be likely are to be used instead.
She is not likely to come so late.
She will probably come to-morrow.