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 What do you have to do and what are you not allowed to do? Fill in must or mustn't. Choose from the drop down menu and form sentences you might hear from your parents and teachers. Do you need help? must, have to, mustn't, needn't. You . tidy up your room. You . smoke The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. Without support for meta transactions in the recipient contract, an externally owned account can not use meta 4Csi.