must have been ...那时一定是 can't have been...那时不可能是
must have been doing意思是 一定一直都在做某事She is so tired .She must has been working all the morning.她很疲劳了她一定工作了一个早上.
must have been doing 表示肯定的猜测
并且猜测这种动作,或者行为一直持续到现在
还有must have been done是must have done的被动形式.
是对过去发生的动作最有把握的猜测,意思是“一定.”