In this sense, must and shall are not related. past tense: He must have seen the dog run out the door. [devoir avoir + participe passé] In legal language, shall is still used to express a binding obligation in a contract or treaty or those kinds of documents and contexts, and must would never be used there in a contract or treaty. Must is not
Must. "Must" is a modal verb most commonly used to express certainty. It can also be used to express necessity or strong recommendation, although native speakers prefer the more flexible form "have to." "Must not" can be used to prohibit actions, but this sounds very severe; speakers prefer to use softer modal verbs such as "should not" or
Must and Have to, Mustn't and Don't Have to. 1. We use must when the speaker thinks it is necessary or important to do an action: You must go. (= It is important that you go.) You mustn't go. Must you go? ~ Yes, I must. 2. We use have to to talk about an action that is necessary because of rules or laws, or because someone obliges us to
Must - English Grammar Today - uma referência à Gramática e uso do inglês escrito e falado - Cambridge Dictionary
Depending on the context, the verb must can be used performatively. The verb have (to) cannot very easily with third person Subjects. What I mean by performatively, is that the actual uttering of the sentence is not a description of the necessity, but a directive, whereby the speaker is exerting their authority over the Subject just by uttering the sentence.
Modal auxiliaries are verbs such as can, must, could, would, etc. which are used with main verbs to express such ideas as possibility, permission, necessity, obligation, etc. 1. Must and have to: Must is used for all persons in the present and future tenses. The negative is must not (mustn't).
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use of must and must not
