核心问题
A:So how did you get into the castle? B:I hid in a farmer's cart which was taking vegetables into the castle.
A: The guards couldn't have been watching vey carefully. [《高中英语教科书》第二册(下), 人教社, p. 55]
[原译]A:……士兵们没有仔细搜查。(《高二英语英汉对照与课后疑难题解析》，陕西师大出版社，p.360) 上句中的 couldn't have been watching very carefully 是“没有仔细搜查”的意思吗？
词典解答
我们在 couldn't have done 诸条中已经讲到,这一结构可用来表示从现在或过去的眼光看来过去不可能发生的事。而 couldn't have been doing 则表示从现在或过去的眼光看来过去不可能一直持续某个动作。例如: She couldn't have been swimming all day. （那时）她不可能整天都游泳。 [句意]……士兵们不可能总是察看得那么仔细。
练习题
请做下面的英语试题：
"You were driving at 100 km an hour, sir." "But officer, I ____. My car can't go more than 80." A. may not have been B. couldn't have been C. wouldn't have been D. needn't have been