为什么must be的否定式是can't be而不是mustn't be

为什么must be的否定式是can't be而不是mustn't be must的否定式是musn't还是mustn't 存在can be/must be ,can’t be 而没有mustn't be can't be ,mustn't be a祈使句的否定式可说成Don't be . Q:that must be a mistake 为什么回答是NO,it can't be.而不是no,it can't can‘t be 和mustn’t be的用法 为什么must的否定式要用can’t或者needn’t?may要用can’t can,can't,must,mustn't,needn't,be allowed to,have to,don't h It must be yours.改为否定句怎么改啊?是用mustn't be还是用can't can't be 和 must’t be can‘t be must’t be区别