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.

And now I must go back to my social duties. (Voynich)

I felt that I had to have the air. (De la Roche)

Baring, because of the type of work in which he was engaged, had been obliged to forego making friends. (Wilson)

Must expresses obligation, necessity, an urgent command or prohibition, and a supposition bordering on assurance.

#1. 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)

This education is indispensable for whatever career you select, and it must not be slipshod or sketchy. (London)

The absence of necessity is expressed by need not:

Must I go there to-morrow? Yes, you must.

No, you needn’t.

#2. arising out of the nature of man and consequently inevitable:

All experience tended to show that man must (Galsworthy)

A command, an urgent (emphatic) request or a In this meaning it is used only with the Indefinite Infinitive.

You must leave the room at once!
You must come to see me every vacation. (Voynich)
You must not speak to a prisoner in a foreign language, madam. (Voynich)

Supposition bordering on assurance, almost a conviction. In this meaning must is used with all the forms of the Infinitive in affirmative sentences only. 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)
Oh, Мае, think how he must be suffering! Poor man! (Webster)
What a comfort you must have been to your blessed mother. (Dickens)
Is she still waiting? She must have been waiting for an hour.

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 tomorrow.