if從句中的動(dòng)詞用過去完成時(shí)，主句中的動(dòng)詞用條件完成時(shí)。這種句子所指的時(shí)間是過去；所提出的條件是不可能實(shí)現(xiàn)的，因?yàn)閕f從句中的動(dòng)作在過去并未發(fā)生。
If I had known that you were coming I would have met you at the air－port．
要是我早知道你要來的話，我就會(huì)在機(jī)場(chǎng)接你了。（但我不知道，所以我也沒去。）
If he had tried to leave the country he would have been stopped at the frontier．
要是他當(dāng)時(shí)試圖離開這個(gè)國家，他就會(huì)在邊界被阻攔。（但他并未這樣做。）
基本形式可能的變化形式
1 可以用could或might而不用would If we had found him earlier we could have saved his life．
要是我們那時(shí)早點(diǎn)找到他的話，我們會(huì)把他救活的。（能力）
If we had found him earlier we might have saved his life．
要是我們那時(shí)早點(diǎn)找到他的話，我們可能會(huì)把他救活的。（可能性）
If our documents had been in order we could have left at once．
要是我們的證明文件都齊備的話，我們那時(shí)就能夠馬上走。（能力或允許）
2 可以用條件完成進(jìn)行時(shí)： 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．
事故發(fā)生的時(shí)候，我坐在車子的后座，因?yàn)闇返男鹤幼谇斑吽纳砼浴Ｒ菧返暮⒆記]坐在那里的話，我就會(huì)坐在前面了。
3 可以在if從句中用過去完成進(jìn)行時(shí)：
I was wearing a seat belt．If I hadn’t been wearing one I’d have been seriously injured．
我當(dāng)時(shí)系了安全帶。要是沒有系上的話，我就會(huì)受重傷了。
4 可以把類型2和類型3結(jié)合起來：
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．
我原想趕乘的那架飛機(jī)墜毀了，所有乘客都遇難身亡。要是我趕上了那架飛機(jī)，我也就沒命了／我就會(huì)摔死了。（類型 3）
If I had worked harder at school I would be sitting in a comfortable office now；I wouldn’t be sweeping the streets．
要是在上學(xué)時(shí)學(xué)習(xí)再努力一些，我現(xiàn)在就會(huì)坐在舒適的辦公室里，而
不會(huì)是在掃大街了。（但我上學(xué)時(shí)不努力，因而我現(xiàn)在在掃大街。）
5 had可以位于句首并省略if： If you had obeyed orders this disaster would not have happened．相當(dāng)于：
Had you obeyed orders this disaster would not have happened．
要是你當(dāng)時(shí)服從了命令，這場(chǎng)災(zāi)難就不會(huì)發(fā)生了。
