If引导的

条件从句 与现在事实相反 从句动词：过去式（be用were）

主句动词：should/would/could/might+do If he were here, he would help us. 与过去事实相反 从句动词：had+done

主句动词：should/would/could/might+have+done If I had been free,I would have visited you. 与将来事实相反 从句动词：过去式/should+动词原形/were+to do

主句动词：should/would/could/might+动词原形 If it should rain tomorrow, we would not go camping. 省略if的虚拟条件句 将虚拟条件从句中的were，had,should放到主语之前，构成主谓倒装 ①Should he come ,tell him to ring me up.

②Were I you,I would not do it.

③Had I been free,I would have visited you. 混合虚拟条件句 不同时间的虚拟：各遵守各的规则 ①If he had listened to me, he would not be in trouble now.

②If he had told me yesterday,I should know what to do now.

③If I were you, I would have gone to her birthday party. 虚拟与陈述的混合：各遵守各的规则 ①He could have passed the exam,but he wasn't careful enough.

②You should have come earlier.The bus left a moment ago. 含蓄虚拟

条件句 but for+名词表示虚拟条件 句子或主句中的谓语动词的形式视具体情况而定。这是考查考生应变能力的最佳试题。 ---But for your timely warning,we ______ into great trouble.

---You know we're friends.

A. would get B. must have got

C. would have got D. can't have got without+名词表示虚拟条件 Without the air to hold some of the sun's heat, the earth at night would be freezing cold. 动词不定式表示虚拟条件 It would be only partly right to follow in this way.

如果用这种方式，仅仅对了一半。 现在分词表示虚拟条件 Having known in time ,we might have prevented the accident.要是及时得知的话，我们也许能阻止这场事故。 过去分词表示虚拟条件 Given more attention, the tree could have grown better.

如果多留心的话，这树本来可以长的更好。 副词otherwise表示虚拟条件 I was too busy at that time.Otherwise,I would have called you.我当时太忙，否则我就给你打电话了。 连词but连接的句子表示虚拟条件 He _____fatter but he eats too little .

A. would become B. would have become

C. must become D. must have become