类别

用法

例句

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+动词原形

If he were here, he would help us.

与过去事实相反

从句动词：had+过去分词

主句动词：should/would/could/might+have+过去分词

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+不定式

主句动词：should/would/could/might+动词原形

If it should rain tomorrow, we would not go camping.

其它状语从句

as if引导的状语从句中动词用过去式或过去完成式

They are talking as if they had been friends for years.

in order that / so that引导的状语从句中动词用can / could / may / might / would等+动词原形

Turn on the light so that we can see it clearly.

宾语从句

demand, suggest, order, insist后接的从句中动词为should+动词原形

He suggested that we not change our mind.

wish后的从句中分别用过去式，过去完成式和should/would+动词原形表示与现在，过去和将来情况相反

I wish I could be a pop singer.

主语从句

在It is necessary / important / strange that…, It is suggested / demanded / ordered / requested that… 等从句中，谓语动词用should+动词原形

It is strange that such a person should be our friends.

其它句型中

It is time that…句型中动词用过去式或should+动词原形

It’s high time that we left.

would rather所接的从句中动词用过去式或者过去完成式

I would rather you stayed at home now.

If only句型中动词常用过去式或者过去完成式，表示强烈的愿望

If only our dream had come true!