状语从句主谓成分的省略，须遵循下面原则：
（一）当从句的主语与主句的主语一致时，从句可以省略主语，从句的谓语动词采用其V-ing结构。例如：
Although working very hard, he failed in the final exam. (Although he worked very hard）
After taking the medicine, she felt much better. (After she took the medicine)
If going there by air, we’ll have to pay twice the fare. (If we go there by air)
（二）当从句的主语和主句的主语一致时，若从句的谓语动词是系动词be或包含be的某种形式，此时从句的主语和be动词通常可以一同省略。常见于以下几种形式:
1、连词+形容词
Whenever (she is) free, she often goes shopping.她有空就去逛商店。
Work hard when (you are) young, or you'll regret.少壮不努力,老大徒伤悲。
2、连词+名词
While (he was) a young boy, he was always ready to help others.他在孩提时代就乐于助人。
3、连词+现在分词
While (she was) walking along the river bank, she was singing a pop song.她沿着河堤边走边唱着流行歌曲。
The boy is running impatiently here and there as if (he is) searching for something lost on the sports ground. 这男孩很不耐心地在操场上到处跑着仿佛在找什么东西。
4、连词+过去分词
If (it is) well read, the book will give you much to think.Unless (they were) asked to answer questions, the students were not supposed to talk in Prof. Li’s class.
5、连词+不定式
He stood up as if (he were) to say something.当时他站起来好像要说什么。
He wouldn't solve the problem even if (he were) to take charge.即使他来负责,他也解决不了这个问题。
6、连词+介词短语
She looked anxious as though (she was) in trouble.她看上去很焦急,好像遇到了麻烦。
He looked everywhere as if (he was)in search of something .他到处看似乎在找什么东西。
（三）若从句的主语和主句的主语不一致，但和主句的宾语一致时，从句中的主语和be动词亦常可省略。例如：
Father advised me not to say anything until (I was) asked.Granny told the children not to talk while (they were) eating.
（四）当从句的主语是it, 且谓语动词是be或包含be的某种形式时, it和be常可同时省略。例如:
If (it is) possible, he will help you out of the difficulty.如果可能的话,他会帮你摆脱困境。
You must attend the meeting unless (it is) inconvenient to you.除非情况对你来说不方便,否则你必须出席这次会议。
扩展资料：
在复合句中，担任状语成分的从句称为状语从句。时间状语从句说明主句动作发生或进行的时间。状语从句由连词引导，其位置通常可以放在句首或句末。放在句首是，常用逗号与主句分开；放在句末时，从句前面往往不加逗号。
引导的时间状语从句的主从复合句中，动作发生在先的用过去完成时，在后的用过去时。表示前面的叙述所没有提到过的信息。它总是对所描述的事件予以引人注目的强调，意为“这时；突然”。
引导时间状语从句的普通类从属连词有when（当……时），while（在……期间），as（当……，一边……一边……），before（在……之前），after（在……之后），since（从……以来），till/until（直到），whenever（无论何时），as soon as（一……就）等。
时间状语从句常用的连词有：when，as，while，before，after，since，till，until，as soon as，once。
参考资料：百度百科-时间状语从句