1、對(duì)將來(lái)情況的虛擬

　　在表示與將來(lái)事實(shí)相反的條件從句中，謂語(yǔ)動(dòng)詞可用should do,were to do或動(dòng)詞的過(guò)去式三種形式表示，但意義略有不同。should表示的可能性最大，動(dòng)詞過(guò)去式的可能性次之，were to do 的可能性最小。

　　主句：主語(yǔ)＋would/should/might/could＋動(dòng)詞原形

　　從句：should＋動(dòng)詞原形/were to＋動(dòng)詞原形。

　　Eg：If he should go to Harvard University, he would make full use of his time.

　　如果他要上哈佛大學(xué)的話，他就會(huì)充分利用他的時(shí)間了。

　　2、對(duì)現(xiàn)在情況的虛擬

　　條件從句中謂語(yǔ)動(dòng)詞用一般過(guò)去式，主句用“would/should/might/could+動(dòng)詞原形”形式，表示對(duì)現(xiàn)在不可能實(shí)現(xiàn)情況的假想。

　　主句：主語(yǔ)＋would/should/might/could＋動(dòng)詞原形

　　從句：動(dòng)詞過(guò)去式

　　Eg：If he studied at this school, he would know the enviornment around the school very much.

　　如果他在這所學(xué)校學(xué)習(xí)的話，他會(huì)很了解周邊環(huán)境。

　　3、對(duì)過(guò)去情況的虛擬

　　條件從句中謂語(yǔ)動(dòng)詞用過(guò)去完成式，主語(yǔ)用“would/could/should/might+have+動(dòng)詞的過(guò)去分詞”形式。

　　提出過(guò)去的未發(fā)生的條件

　　主句：主語(yǔ)＋would/should/might/could have＋動(dòng)詞過(guò)去分詞

　　從句：had＋動(dòng)詞過(guò)去分詞

　　Eg：If I had read the book, I would have told you about it.