In this free must and mustn't speaking activity, students write and discuss how they would like people to behave in a perfect world using the modal verbs must for obligation and mustn't for prohibition. Working alone, students complete sentences about how they would like different people to behave using must and mustn't. Students then discuss RFC 2119 gives the following definitions: MUST This word, or the terms " REQUIRED " or " SHALL ", mean that the definition is an absolute requirement of the specification. MUST NOT This phrase, or the phrase " SHALL NOT ", mean that the definition is an absolute prohibition of the specification. SHOULD This word, or the adjective " RECOMMENDED 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. a. You must attention at school. to pay / pay / paying b. You must quiet in the library. be / to be / is c. You mustn't across the road. running / run / ran d. We shout in class. mustn't to / must no / mustn't e. She help her brother with his homework. must to / must / musts f. You must after your pets. looking / to look / look g. Differences. Both ' must ' and ' need ' are used to talk about a sense of necessity or obligation. However, 'must ' is used to express a strong sense of obligation or necessity. It suggests that something is required or necessary and that there is no other option. For example: must have + past participle when we draw the conclusion that something DID happen. Sheila got a tan. She must have spent a lot of time in the sun lately. There was one banana left, but now it's gone. My husband must have eaten it. must not have + past participle when we draw the conclusion that something did NOT happen. The car is still dirty. .

use of must and must not