if（only虚拟语气的三种时态）

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；

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；

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

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

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

虚拟语气的时态

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?