非真实条件句/if 条件句 主句 had+过去分词 would{should, could, might} +have done 如：If he had seen you yesterday, he would have asked you about it.

If you had come earlier, you might have met him.

If I had known your telephone number, I would have called you.

If I hadn't taken your advice, I would have made a bad mistake.

You wouldn't have caught a cold if you had put on more clothes.

3. 表示和将来事实相反，谓语动词的主要形式如下：

非真实条件句/if 条件句 主句 should /were to+ 动词原形

动词的过去时 would{should, could, might} +动词原形 如：If it should rain, the crops would be saved.

If it were to snow tomorrow, they wouldn't go out.

If I were to do it, I would do it in a different way.

If he got there this afternoon, he would buy a map of the city.

If I should be asked/were asked /were to be asked to go, I'd certainly go.

特别注意：

1. 如非真实条件句的谓语含有系动词were或助动词had或shoul