cannot have done/could not have done

cannot have done 与 could not have done 都可以表示以现在的眼光看来“过去不可能发生的事”，如句(a)，但是 could not have done 还可以表示从过去的某个时间点看来“以前不可能发生的事”，如句(b)。另外，在句(a)中用 cannot have gone 比用 could not have gone 语气要强一些。还要注意一点，could not have done 有其相对应的肯定式，即 could have done，而 cannot have done 却没有其相对应的肯定式，也就是说 can + 完成体不定式只能用在否定句和疑问句中。你可以从下面的例句中发现它们之间的相同处和相异处（参见 could have done, couldn't have done）。

1. He can't have taken it upstairs this morning! (Galsworthy) 他今天早上是不可能把它带上楼的！

2. Where can he have gone? (Cronin) 他能去哪了呢?

3. She's two hours late—what can have happened? (PEU) 她已经晚了两个小时了,可能发生了什么事呢?

4. I do not see how I could have done otherwise. (Wells) 我看不出当时我还能用其他方式来做那件事。

5. They couldn't have left so soon. (Mis) 他们不可能那么早就走了。