条件从句分为：真实条件句和虚拟条件句。

虚拟条件句又包括一种特殊情况就是混合虚拟条件句。

真实条件句：当从句表达的条件是有可能实现的、有可能是真实的条件
从句中用一般现在时代替一般将来时，用现在完成时代替将来完成时
例如：You can arrive in Beijing earlier for the meeting provided you don ’t mind taking the night train.（从句遵循“退时态”原则，在非真实条件句中这条原则亦适用） 虚拟条件句：当从句中表达的条件是不可能实现的、与客观事实完全相反。

特殊情况：should,had,were 省略if 提前。

（倒装）
例如：If you should need more information ，please let me know 。

Should you need more information,…… If you had finished your homework,…… Had you finished your homework ，…… If I were you,…… …… 混合虚拟条件句
意即从句和主句中的动作不同时发生
Eg ：If he had followed my advice,he would be quite all right now.
If China had not been liberated,the working people would still be leading a miserable life.
虚拟语气的几种情况
表示“要求、建议、命令”等动词后面用should （可省略）+动词原形 两种常用词：insist 1.坚持认为（后面用should+动原）
2.暗示，表明（该用什么时态就用什么时态) Suggest 1.表明，暗示（不用虚拟） 2.建议
Eg.He suggested that a meeting be held to discuss the matter.
The look on his face suggested that he was quite satiesfied with what I had done for him.
Would rather+从句
从句中的谓语动词用过去形式表虚拟。

Eg. I would rather you did this instead of me.
主句中的虚拟语气
It be+adj（necessary、important、essential……）+that+……（should）+动词原形
It be+过去分词（suggested、requested、proposed）+that+……should+动词原形（用于此结构的过去分词表示请求、建议、命令）
Eg.It is desired that the building of the house be completed next month.
It is (high) time that……（should）+动词原形
倒装重点
一、条件状语从句
强调词only
Eg:I will go there only if I have time.
Only if I have time will I go there.
二、时间状语从句
Eg:I will go there when I have time.
Only when I have time will I go there.
三、as引导的让步状语从句
Hard as/though he works,he makes little progress.
=Though he works hard,he makes little progress.
Child as/though he was,he knew what was the right thing to do.
=Though he was a child,……。