我们看下面的句子：
- My wallet might have been stolen by that little boy.
- John must have finished his homework because he is playing basketball.
上面的句子其形式是：情态述词 + have + 完成分词，这种结构用来猜测过去做过某事。用 might 的话，说话者对自己的猜测不是很肯定；用 must 的话，说话者对自己的猜测相当有把握。
3. We could have stayed there for one more week but we had spent all the money.
以上 could + have + 完成分词，这种结构可以用来表达“本应该做某事，但是实际上没有做”。
4. He could have forgotten what he did.
以上 could + have + 完成分词，用来表达“可能有做过(发生过)某事”，说话者不是十分确定该事情过去是否确实发生。
所以，对 could + have + 完成分词，要具体分析语境，才能知道它究竟是表示猜测，还是表示虚拟。
更多例句：
3. He must have been strong when he was young.
4. It can be and should be so applied that questions asked by the teacher could have occurred to the student himself.