That’s surprising. Usually she’s a very cautious driver and obeys all the traffic laws. She must not have seen the sign.
这个句子中的 must not have seen用得对吗?这里显然是表示推测耶,怎么用must的否定式呢?
在美国英语中,表推测的must也可用于否定句的,此时的 must not=cannot:
That’s surprising. Usually she’s a very cautious driver and obeys all the traffic laws. She must not (=can't) have seen the sign. 真是让人吃惊。她通常是很小心驾驶的,而且会遵守所有交通规则。她一定是没有看到那个标志。