首先if條件從句知識溫故而知新：
（1）一般普通假設，不屬于虛擬語(yǔ)氣。If you study hard now, you will be a successful man.
if 引導從句為一般現在時(shí)，主句為將來(lái)時(shí)。
（2）與現在事實(shí)相反的假設，虛擬語(yǔ)氣。If I were you, I would join them.
if 引導從句為一般過(guò)去式，主句為would/could/should/might 動(dòng)詞原形。
（3）與過(guò)去事實(shí)相反的假設，虛擬語(yǔ)氣。If I had got there earlier, I should have met her.
if 引導從句為過(guò)去完成時(shí)，主句為would/could/should/might have 動(dòng)詞過(guò)去分詞。
我們今天著(zhù)重學(xué)習的內容是if 引導從句為過(guò)去完成時(shí)，表示與過(guò)去事實(shí)相反的假設，虛擬語(yǔ)氣；主句為would/could/should/might have 動(dòng)詞過(guò)去分詞。
【什么是與過(guò)去事實(shí)的相反虛擬語(yǔ)假設呢？】
有一件事發(fā)生在過(guò)去，這件事已經(jīng)成為事實(shí)，現在我們假設在過(guò)去的時(shí)候，又另外一種情況出現的話(huà)，又會(huì )有什么樣的結果。那么由if引導的從句就是這種與過(guò)去事實(shí)相反的虛擬或假設。
if 從句是一種虛擬的條件或假設，和過(guò)去事實(shí)相反的。
主從句的時(shí)態(tài)具體如下：
從句： had 動(dòng)詞過(guò)去分詞
主句：would/could/should/might have 動(dòng)詞過(guò)去分詞
（1）If I had remembered the answer, I should have got better grade.
譯文：如果我記得答案，我應該會(huì )得到一個(gè)更好的分數。
從句：If I had remembered the answer，過(guò)去完成時(shí)（had 動(dòng)詞過(guò)去分詞），had remembered ~
主句： I should have got better grade，would 現在完成時(shí)（have 動(dòng)詞過(guò)去分詞），would have got~
分析：首先考試這件事情發(fā)生在過(guò)去，并且我的成績(jì)考得不好也是事實(shí)。這里對過(guò)去進(jìn)行假設，假設另外一種情況——我記得那個(gè)答案，推測假設的結果——得到更高一點(diǎn)的分數。if 引導的對過(guò)去事實(shí)假設的虛擬語(yǔ)氣。
（2）If he had taken my advice, he would not have made such a mistake.
譯文：如果他采納了我的建議，他就不會(huì )犯這樣的錯誤。
從句：If he had taken my advice，過(guò)去完成時(shí)（had 動(dòng)詞過(guò)去分詞），had taken ~
主句： he would not have made such a mistake，would 現在完成時(shí)（have 動(dòng)詞過(guò)去分詞），would have made ~
分析：事情發(fā)生在過(guò)去，事實(shí)上他沒(méi)有采取我的建議?，F在假設另外一種情況——他采取了我的建議，那么結果——他不會(huì )犯這樣的錯誤。對過(guò)去客觀(guān)存在的事實(shí)一種假設，以及推測其可能發(fā)生的結果。if 引導的對過(guò)去事實(shí)假設的虛擬語(yǔ)氣。
練習（答案在文末）
If he ____ ( study ) harder, he might have passed the exam.
If there ____ (be ) a heavy snow, we ____ (not go )skating.
If I ____ (get ) there earlier , I ____ (meet ) her.
經(jīng)典例句：
If I were you, Mrs Gretchen, I just wouldnt worry about it
要是我是你，格蕾琴夫人，我一點(diǎn)都不會(huì )擔心。
If you went into town, youd notice all the pubs have loud jukeboxes
如果你進(jìn)了城，你會(huì )注意到所有的酒吧都有音量很大的自動(dòng)點(diǎn)唱機。
If he improved his IT skills, hed (= he would) easily get a job.
他如果提高了自己的信息技術(shù)技能，就會(huì )容易找到工作。
You would know what was going on if youd (= you had) listened.
你若是注意聽(tīng)了，就會(huì )知道發(fā)生什么事了。
They would have been here by now if theyd caught the early train.
假若他們趕上了早班火車(chē)，現在就該到這里了。
If only you had told me that some time ago
要是你早些時(shí)候告訴我該多好?。。ㄖ苯訉^(guò)去的事情進(jìn)行虛擬或假設）
《柯林斯高階英漢雙解學(xué)習詞典》
答案：
had studied,如果他學(xué)習努力一點(diǎn)，他就有可能通過(guò)這個(gè)考試。
had been , should not have gone ，如果這里有雪，我們就不應該在這里滑雪。
had got , would have not，如果我早一點(diǎn)到那里，我可能會(huì )遇到她。