if句又叫条件句,表示“假如”,是状语从句的一种,是一种非常常见的从句形式,用来提出一种与客观现实不相符或根本不可能存在的条件,拥有三种形式。
①提出将来会发生的事:主句:主语+would/should/might/could+动词原形
从句:should+动词原形/were to+动词原形
eg. If he should go to Harvard University, he would make full use of his time. 如果他要上哈佛大学的话,他就会充分利用他的时间了。
②提出现在的条件: 主句:主语+would/should/might/could+动词原形从句:动词过去式
eg. If he studied at this school, he would know the enviornment around the school very much.如果他在这所学校学习的话,他会很了解周边环境。
③提出过去的未发生的条件:主句:主语+would/should/might/could have+动词过去分词
从句:had+动词过去分词
eg. If I had read the book, I would have told you about it.我如果看过本书,我会把内容告诉你了。
总结一下,也就是说:
与现在事实相反,虚拟语气的条件从句的谓语用过去式。如:If I knew her number I could ring her up. 要是我知道她的电话号码,我就可以给她打电话了。
与过去事实相反,虚拟语气的条件从句的谓语用过去完成时。如:If I'd left sooner,I'd have been on time. 要是我早点动身,我就准时到了。
与将来事实相反,条件从句的谓语用过去式。如:If he went,would you go too? 如果他去,你也去吗?