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.
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.