1.对现在虚拟：条件句：一般过去时主句：could/would/+动词原形。2.对过去虚拟：条件句：过去完成时主句：could/would have done。3.对将来虚拟：条件句：1.were to do；2.should do；3.一般过去时：主句could/would/+动词原形。

一、对现在情况的虚拟条件从句中谓语动词用一般过去式，主句用“would/should/might/could+动词原形”形式，表示对现在不可能实现情况的假想。If I were you,I would choose to work in a small town.如果我是你，我会选择在小城镇工作。（事实上我不是你）If I had a map now,we could easily get out of the forest.如果现在有地图，我们能很容易地走出森林。（事实上是现在没有地图）注意：在条件从句中，如果谓语动词是be的形式，不管主语是单数还是复数，书面语中用were,而口语中如果是单数，也可以用was,不过在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，我们如何能尝到创造成功的快慰？

二、对过去情况的虚拟条件从句中谓语动词用过去完成式，主语用“would/could/should/might+have+动词的过去分词”形式。If it had not been for your help yesterday,I could not have caught the bus.如果昨天没有你的帮助，我不会赶上公交车。（事实是你昨天已经帮助了我，所以我赶上了车)

三、对将来情况的虚拟在表示与将来事实相反的条件从句中，谓语动词可用should do,were to do或动词的过去式三种形式表示，但意义略有不同。should表示的可能性最大，动词过去式的可能性次之，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，人人皆可成为聪明人。