同学你好，该知识点来自沪江网校《英语零基础直达职场全能【学霸班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
They can't have been butchers.
They must have been bakers.
can't/must have+过去分词，都是情态动词的特殊用法，是虚拟语气，不是某一种特定的时态。
can't have+过去分词，用于表示否定的推测，“不可能……”
From her knowledge of Douglas's habits, she feels sure that the attacker can't have been Douglas.
根据她对道格拉斯习性的了解，她敢断定他不可能是那个袭击者。

must have+过去分词，表示对过去的肯定推测，“肯定……”
She must have misheard.
她一定是听错了。

祝同学学习进步~