must be...mast be是“准确，无误”的意思，实际上表达一种推测，只是推测的准确率比较高，那么就可以用must be来表示。

比如说He must is our new English teacher. 可以直接翻译为他就是我们新的英语老师。用于推测现在或将来发生的情况。例如：He must be out of his mind to say that. 他这样说一定是精神不正常。I must look funny in this hat ! 我戴这顶帽子看起来一定很滑稽。

2. must have been...用于推断过去或现在已经发生的事。例如：You must have seen him yesterday. 你昨天一定看见他了。It must have been the coffee that kept me awake. 一定是喝了咖啡，使我睡不着。