Must is commonly used to express certainty or necessity. It can also express strong recommendation, though native English speakers prefer the more flexible "have to". "Must not" is used to prohibit actions but sounds very severe; "should not" or "ought not" are softer alternatives to dissuade rather than prohibit. Examples are given for the different uses of must and must not. Guidelines are provided on the positive and negative forms of must and must not versus have to in different tenses. The key difference between "must not" and "do not have to" is that the former prohibits an action while the latter means an action is not required.