在《薄冰英语惯用法词典》must 词条中有如下内容：表示合乎逻辑的推断和假定，must 意思是“谅必是”、“一定是”， ......
1. must be...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。