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
Though understanding no Chinese, the American girl was able to communicate with the other students in her class.
When asking the teacher, he was very polite.
Until finished, the problem was a continuous worry to me.
He’ll go to the seaside for his holiday if (it is) possible.
一、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。
She hurried out of the room, as if (she was) angry.
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。
He could write poems when (he was) yet a child.
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。
He came across the picture while (he was) on a visit to New York.
（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。
二、从状语从句的类别看，此种省略常见于一下几种情形
（一）时间状语从句
常用于由when, while, before, after, whenever, till, until, since, once, as soon as等引导的时间状语从句。
When (she was) very young, she began to learn to play the piano. 她很小时，就开始学习弹钢琴。
When arriving, send me a telegram. (When you arrive, send me a telegram.) 到达之后，来个电报。
While (I was) at college, I began to know him, a strange but able student. 我在上大学时就开始认识他，一个奇怪但有能力的学生。
Before leaving, turn off all the lights. (Before you leave, turn off all the lights.) 走之前，请关闭所有的灯。
Whenever (it is ) possible, you should come and help. 不管什么时候只要有可能就来帮忙。
Don’t come in until (you are) asked to. 不叫你请你不要进来。
You should let us know the result as soon as (it is) possible. 你应尽快让我们知道结果。
（二）让步状语从句
常用于由though, although, even if, even though, while, whether…or…等引导的让步状语从句。
He is a good man, though sometimes (he is) rather dull. 他真是个好人，尽管有时有点无聊。
Although (he was) frightened, he managed to ran away.
Even if (I am ) invited to, I won’t go to such a bad lecture. 即使邀我去，我都不想听如此坏的报告。
Her opinion, whether (it was) right or wrong, deserved our attention.
（三）条件状语从句
常用于由if, unless等引导的条件状语从句。
Send the goods now if (they are) ready. 货物如果准备好了，请送过来。
He will come if (he is) asked. 如果叫他来，他就来。
If (it is) necessary, ring me at home. 如果有可能，朝我家里打电话。
Come along with me if (it is) possible. 如果有可能和我一起去吧.。
If (it is) true, this will cause us a lot of trouble. 如果是真的，这会给我们带来很多麻烦。
He won't go there with us unless (he is) invited.除非受到邀请,否则他不会和我们一道去那里。
You should stay where you are, unless (you are) asked to leave. 你应呆在你现在的地方不动，除非叫你动。
（四）方式状语从句
常用于由as if, as though等引导的方式状语从句。
He acts as if (he were) a fool. 他的行为举止好象是个傻子。
He opened the desk, as if (he was) in search of something important. 他打抽屉仿佛要找一件什么重要的东西。
The football player is rolling on the ground as if (he is) hurt badly in the leg. 足球运动员在操场上打着滚似乎他的腿伤得厉害。
（五）比较状语从句
常用于以下结构。
The exhibition is more interesting than (it was) expected .这次展览比被预料的有趣的多
She has finished the work earlier than (it has been ) expected. 她这项工作比预料的提前完成。
（六）地点状语从句
常用于where(ver) possible, where(ver) necessary等结构：
Put in articles wherever necessary in the following passages. 在下列文章中需要的地方填入冠词。
和主句都是he，那么可以省掉when后面的he，但是同时要改变从句的谓语，变成非谓语，是be动词的话，
直接省略，上句可改为when asked where to go ，he kept silent
如果从句谓语不是be动词，就要根据从句谓语与主语的关系相应变成现在分词和过去分词，如：
once you find the truth，you will feel surprised=once finding the truth，you will feel surprised本回答被网友采纳