A if從句中的動詞用過去完成時，主句中的動詞用條件完成時。這種句子所指的時間是過去;所提出的條件是不可能實現的，因為if從句中的動作在過去并未發生。

　　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，我就會在機場接你了。(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.

　　要是他當時試圖離開這個國家，他就會在邊界被阻攔。(但他并未這樣做。)

　　B 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.

　　要是我們那時早點找到他的話，我們會把他救活的。(能力)

　　If we had found him earlier we might have saved his life.

　　要是我們那時早點找到他的話，我們可能會把他救活的。(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，我們那時就能夠馬上走。(能力或允許)

　　2 可以用條件完成進行時：

　　At the time of the accident I was sitting in the back of the car，because Toms little boy was sitting beside him in front.If Toms boy had not been there I would have been sitting in front.

　　事故發生的時候，我坐在車子的后座，因為湯姆的小兒子坐在前邊他的身旁。要是湯姆的孩子沒坐在那里的話，我就會坐在前面了。

　　3 可以在if從句中用過去完成進行時：

　　I was wearing a seat belt.If I hadnt been wearing one Id have been seriously injured.

　　我當時系了安全帶。要是沒有系上的話，我就會受重傷了。

　　4 可以把類型2和類型3結合起來：

　　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墜毀了，所有乘客都遇難身亡。要是我趕上了那架飛機，我也就沒命了/我就會摔死了。(類型 3)

　　If I had worked harder at school I would be sitting in a comfortable office now;I wouldnt be sweeping the streets.

　　要是在上學時學習再努力一些，我現在就會坐在舒適的辦公室里，而

　　不會是在掃大街了。(但我上學時不努力，因而我現在在掃大街。)

　　5 had可以位于句首并省略if：

　　If you had obeyed orders this disaster would not have happened.相當于：

　　Had you obeyed orders this disaster would not have happened.

　　要是你當時服從了命令，這場災難就不會發生了。

　　A if從句中的動詞用過去完成時，主句中的動詞用條件完成時。這種句子所指的時間是過去;所提出的條件是不可能實現的，因為if從句中的動作在過去并未發生。

　　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，我就會在機場接你了。(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.

　　要是他當時試圖離開這個國家，他就會在邊界被阻攔。(但他并未這樣做。)

　　B 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.

　　要是我們那時早點找到他的話，我們會把他救活的。(能力)

　　If we had found him earlier we might have saved his life.

　　要是我們那時早點找到他的話，我們可能會把他救活的。(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，我們那時就能夠馬上走。(能力或允許)

　　2 可以用條件完成進行時：

　　At the time of the accident I was sitting in the back of the car，because Toms little boy was sitting beside him in front.If Toms boy had not been there I would have been sitting in front.

　　事故發生的時候，我坐在車子的后座，因為湯姆的小兒子坐在前邊他的身旁。要是湯姆的孩子沒坐在那里的話，我就會坐在前面了。

　　3 可以在if從句中用過去完成進行時：

　　I was wearing a seat belt.If I hadnt been wearing one Id have been seriously injured.

　　我當時系了安全帶。要是沒有系上的話，我就會受重傷了。

　　4 可以把類型2和類型3結合起來：

　　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墜毀了，所有乘客都遇難身亡。要是我趕上了那架飛機，我也就沒命了/我就會摔死了。(類型 3)

　　If I had worked harder at school I would be sitting in a comfortable office now;I wouldnt be sweeping the streets.

　　要是在上學時學習再努力一些，我現在就會坐在舒適的辦公室里，而

　　不會是在掃大街了。(但我上學時不努力，因而我現在在掃大街。)

　　5 had可以位于句首并省略if：

　　If you had obeyed orders this disaster would not have happened.相當于：

　　Had you obeyed orders this disaster would not have happened.

　　要是你當時服從了命令，這場災難就不會發生了。