if从句中的动词用过去完成时，主句中的动词用条件完成时。这种句子所指的时间是过去；所提出的条件是不可能实现的，因为if从句中的动作在过去并未发生。
If I had known that you were coming I would have met you at the air－port．
要是我早知道你要来的话，我就会在机场接你了。（但我不知道，所以我也没去。）
If he had tried to leave the country he would have been stopped at the frontier．
要是他当时试图离开这个国家，他就会在边界被阻拦。（但他并未这样做。）
基本形式可能的变化形式
1 可以用could或might而不用would：
If we had found him earlier we could have saved his life．
要是我们那时早点找到他的话，我们会把他救活的。（能力）
If we had found him earlier we might have saved his life．
要是我们那时早点找到他的话，我们可能会把他救活的。（可能性）
If our documents had been in order we could have left at once．
要是我们的证明文件都齐备的话，我们那时就能够马上走。（能力或允许）
2 可以用条件完成进行时：
At the time of the accident I was sitting in the back of the car，because Tom’s little boy was sitting beside him in front．If Tom’s boy had not been there I would have been sitting in front．
事故发生的时候，我坐在车子的后座，因为汤姆的小儿子坐在前边他的身旁。要是汤姆的孩子没坐在那里的话，我就会坐在前面了。
3 可以在if从句中用过去完成进行时：
I was wearing a seat belt．If I hadn’t been wearing one I’d have been seriously injured．
我当时系了安全带。要是没有系上的话，我就会受重伤了。
4 可以把类型2和类型3结合起来：
The plane I intended to catch crashed and everyone was killed．If I had caught that plane I would be dead now／I would have been killed．
我原想赶乘的那架飞机坠毁了，所有乘客都遇难身亡。要是我赶上了那架飞机，我也就没命了／我就会摔死了。（类型 3）
If I had worked harder at school I would be sitting in a comfortable office now；I wouldn’t be sweeping the streets．
要是在上学时学习再努力一些，我现在就会坐在舒适的办公室里，而
不会是在扫大街了。（但我上学时不努力，因而我现在在扫大街。）
5 had可以位于句首并省略if：
If you had obeyed orders this disaster would not have happened．相当于：
Had you obeyed orders this disaster would not have happened．
要是你当时服从了命令，这场灾难就不会发生了。