【问】 I would have come sooner but I____that they were waiting.
 A. haven’t known B. hadn’t know
 C. don't know D. didn't know
 在一般的非真实条件句中，表示虚拟语气的谓语动词的时态是相互呼应的。例如：
 If I had married her，I would have been unhappy.
 要是那时我跟她结了婚的话，那我是很不幸的。
 但是，上面试题的正确答案为D(didn't know)，这是为什么呢?
 【答】 首先应弄清这种句型的特点：but或but that引出的分句描述的是真实情况，but前的分句描述的是非真实的情况(假想的情况)。but句所描述的情况阻碍了前面的句子的情景得以发生或实现。换言之，but句的反面意义便是一个虚拟条件(倘若不)。
 先看几个实例：
 1. Franklin would gladly have spent the rest of his days in quiet study and research. But hewas a very important person now. and the country needed him for public service.
 富兰克林本来是会在安静的研究中愉快地渡过他的后半生的，但是他当时是一个非常重要的人物，国家需要他担任公职。
 2. We would have avoided making such a mistake，but we didn't follow his sound advice.我们本来可以不犯这样的错误，只是我们没有接受他的忠告(倘若我们接受了他的忠告，我们本来是可以避免这样的错误的)。
 3. They might have left earlier，but the car wasn't ready.
 要是汽车准备好了的话，他们本可能早点儿离开的。
 4. I would have written before but I have been ill.(Yeats )
 要不是我病了的话，我会在此之前写信的。
 由这些实例可以很容易理解，为什么在but句中使用陈述语气，而在它前面的句子里使用虚拟语气了。
 请做下面的试题：
 1. I________ abroad but I had no money.
 A. would go B. would have gone
 C. would be going D. will have gone
 2. “What happened to the flower I gave you?”
 “The flower________ well，but I did not water it.”
 A. would grow B. might grow
 C. would have grown D. should have grown
 (参见but that)