1.對現(xiàn)在虛擬：條件句：一般過去時主句：could/would/+動詞原形。2.對過去虛擬：條件句：過去完成時主句：could/would have done。3.對將來虛擬：條件句：1.were to do；2.should do；3.一般過去時：主句could/would/+動詞原形。

一、對現(xiàn)在情況的虛擬條件從句中謂語動詞用一般過去式，主句用“would/should/might/could+動詞原形”形式，表示對現(xiàn)在不可能實現(xiàn)情況的假想。If I were you,I would choose to work in a small town.如果我是你，我會選擇在小城鎮(zhèn)工作。（事實上我不是你）If I had a map now,we could easily get out of the forest.如果現(xiàn)在有地圖，我們能很容易地走出森林。（事實上是現(xiàn)在沒有地圖）注意：在條件從句中，如果謂語動詞是be的形式，不管主語是單數(shù)還是復數(shù)，書面語中用were,而口語中如果是單數(shù)，也可以用was,不過在if I were...中一般用were.If the world were perfect,how could we enjoy the pleasure and comfort of being successful?世界若完美，我們如何能嘗到創(chuàng)造成功的快慰？

二、對過去情況的虛擬條件從句中謂語動詞用過去完成式，主語用“would/could/should/might+have+動詞的過去分詞”形式。If it had not been for your help yesterday,I could not have caught the bus.如果昨天沒有你的幫助，我不會趕上公交車。（事實是你昨天已經幫助了我，所以我趕上了車)

三、對將來情況的虛擬在表示與將來事實相反的條件從句中，謂語動詞可用should do,were to do或動詞的過去式三種形式表示，但意義略有不同。should表示的可能性最大，動詞過去式的可能性次之，were to do 的可能性最小。If things were to be twice，all would be wise.假如凡事都可以重新做一次，人人皆可成為聰明人。