can't be和mustn'be的区别(急)

为什么不用mustn',从来不能选mustn'be? 例题?请解释下,难道没有mustn'.can'be吗:B 为什么呢;be的区别. A.mustn',谢谢 我认为这是十分肯定的事;be B;t be 答案,我不懂;t be和mustn'tbe:Li Hong _____ be in shanghai because I saw him in BeiJing only 10 minutes agocan',我现在初三做的题目为什么都用can'

mustn'be？
是mustn’t be吧……
我记得它不是推测用法
否定的推测用can't be
mustn’t be有个意思是禁止

没这种用法

语气强调：you mustn't enter that door, it is forbidden. (命令式) 一般语气：you can't smoke, it is not good for your health.（劝慰式） 表示能力的否定：i can't play piano, i haven't learn it before. he lost his sight. 他失明了 it's a good scene by the lake, i'm very satisfied. 湖边景色很美。 it's my point of view that you are wrong. 我认为你错了。