英语状语从句中常见省略现象，主要发生在主从句主语一致或从句主语为it且主要动词为be的情况。省略形式常见于条件、让步、时间、方式和比较状语从句中。此外，当从句主语和主句主语一致时，有时也可以省略从句谓语。注意，当从句主语和主句主语不一致时，从句需用完全形式或独立主格结构表达。
英语状语从句中的省略现象经常见，如果不了解省略原则，有时就不能理解句子的意思。特别是在口语中，有时为了使语言精炼，达到言简意赅的效果, 人们常将状语从句进行"简化"。
一、牢记省略的两个必备条件
状语从句具备下列两个条件，从句中的主语和be动词常可省略。:
1、主句和从句的主语一致，或从句主语为it；
2、从句主要动词是be的某种形式。
例如：
When ( the museum is ) completed , the museum will be open to the public next year .
这座博物馆建成后，将于明年向公众开放。
He'll go to the seaside for his holiday if (it is ) possible.
如果可能的话，他将去海边度假。
另外，比较状语从句经常省略be动词以及相比较的形容词或副词。
例如：
I'm taller than he (is tall ).
我比他个子高。
The higher the temperature (is), the greater the pressure (is ).
温度越高， 压力越大。
二、常见省略现象的句式
状语从句的"简化"现象常存在于以下五种句式中：
当状语从句的主语是it，且谓语动词是be时，it和be要完全简化掉。
1、由 if 和 unless等引导的条件状语从句；
（1）、连词+形容词
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。
（2）、连词+过去分词
例如：
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。
当状语从句的主语和主句的主语一致时，从句可以将主语和be动词简化掉。
2、由although, though, even if / though等引导的让步状语从句；
（1）、连词+名词
Although (he was) a farmer, now he is a famous director.
尽管他曾是个农民，而现在是位著名的导演了。
（2）、连词+现在分词
例如：
Although (he is) doing his best in maths these days, he has still got no good marks.
尽管近来他一直在学数学，但他仍然没有取得好成绩。
（3）、连词+形容词
例如：
Although (he was) the youngest in the group, Tom won all the prizes.
尽管汤姆是队里年级最小的， 但是他赢得了所有奖项。
3、由when、while、 as、 before、after、 until / till等引导的时间状语从句;
（1）、连词+形容词
例如：
As (he was) young，he learned how to ride a bike.
他小时候就学会了骑自行车。
Work hard when (you are) young，or you'll regret.
少壮不努力，老大徒伤悲。
（2）、连词+介词短语
例如：
He had mastered the English language before (he was) in the USA.
他到美国之前就懂英语了。
（3）、连词+过去分词
Fear is something innate that we actually rely on to keep us safe when (we are) faced with danger.
人的恐惧是与生俱来的，当我们面临危险时，实际上我正因为恐惧才让我们保持安全。
（4）、连词+现在分词
When (the brain is) multitasking, the brain is concentrating on one task and then switching to the other, in sequence.
当大脑执行多项任务时，往往是按照顺序，集中精力完成一项，然后再转移到另一项任务上。
4、由 as 和 as if 等引导的方式状语从句;
（1）、连词+不定式
例如：
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。
（2）、连词+过去分词
例如：
He did it as required.
他按要求做完了。
（3）、连词+形容词
例如：
The girl looked as if afraid of nothing.
那个女孩看上去什么都不怕。
5、由as, than等引导的比较状语从句。
（1）连词+过去分词
例如：
The concert was a great success than (it was) expected.
这场音乐会出乎意料地取得了巨大成功。
（2）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。
三、其他形式
1、 在状语从句中，从句主语和主句主语一致，有时候从句谓语没有系动词，也可以省略。
例如：
When comparing different cultures, we often pay attention only to the difference without noticing many similarities.
当我们在比较不同文化时，我们经常只关注于它们的差异性，而不注意那些相似点。
（这里when comparing different cultures就是when we compare different cultures。）
when doing/while doing已经成固定用法，不必拘泥于主语是否一致，例如：
Good posture is essential when (we are) working at the computer.
用电脑工作时良好的姿势极其重要。
从句主语与主句主语一致，谓语动词不是be动词，则将主语省略，谓语动词变为doing/done
Though he knows the truth, he remained silent. 省略为：
Though knowing the truth, he remained silent. 虽然他知道实情，却保持沉默。
2、 由if引导的习惯用法，常有：if not，if possible，if necessary 等。
例如：
Correct the mistake in your composition if any.
请改正文章中的错误。
（这里的if any 相当于 if there is any。）
注意：当从句主语和主句主语不一致时，从句部分要么用完全形式；要么用独立主格结构来表达。
例如:
When the meeting was over, all the people went out of the meeting-room.
当会议结束时，人们都走出了会议室。
分类具体如下注意：
1）
--when/while/once/before/after 时间状语从句的省略
when+doing/done/介词短语/形容词
Be careful when (you are) crossing the road. 过马路时小心。
The review would then be made public when (it is) completed. 审查工作完成后将向公众公布。
When (you are) in Rome, do as the Romans do. 入乡随俗。
When (it is) necessary, instinct is the most reliable resource you can fall back on.
必要时，本能是你可使用的最可靠的资源。
while+doing/介词短语
While (I was) doing the dishes, I listened to radio. 我洗碗的时候，会听收音机。
The two ministers have yet to meet, but may do so while (they are) in New York.
两位部长还未会面，但他们在纽约期间可能会见面。
once done/介词短语
Once (they are) completed, they would cut transport costs by up to 25%.一旦完工，它们将减少多达25%的运输成本。
Once (he is) on the open road, he let rip. 一上空旷的公路，他就全速行驶。
before/after+doing (此处可以将after/before理解为介词)
The doctor may seek the concurrence of a relative before carrying out the procedure.
医生可以征得亲属的同意后再施行此项手术。
Two children drowned after falling into the river. 两个孩子掉进河里淹死了。
2）--where地点状语从句的省略
The Roads Service is continuing to salt roads where (it is) needed on Saturday.
周六，道路服务部门将继续在需要的道路上撒盐。
Keeping moral and religious demands, where possible, out of politics. 在可能的情况下，将道德和宗教需求排除在政治之外。
All the rigging had been examined, and renewed where necessary. 所有的索具都检查过了，必要的地方都换了新的。
3）--If, unless条件状语从句的省略
If+doing/done/形容词
If using a wok with metal handles, be sure to use pot holders when touching the handles. 如果使用的是有金属把手的锅，在接触把手时一定要使用锅托。
If done correctly, leveraging your reputation can accomplish that goal for you. 如果做得好，利用你的声誉可以为你实现这个目标。
If necessary I can come at once. 如果有必要，我可以马上来。
unless+doing/done/介词短语
He won’t answer unless knowing that he will not be identified. 除非知道他的身份不会被发现，否则他不会回答。
Unless invited to speak, you should remain silent at the conference. 除非被邀请讲话，否则你在会上应保持沉默。
He never cries unless in dreams. 他从不哭，除非在梦中。
4）--though/although让步状语从句的省略
though/although+doing/done/介词短语/形容词
Though lacking money, his parents managed to send him to university. 虽然缺钱，他的父母还是设法送他上了大学。
Although knowing that his family objected to his drinking, he would drink a glass of wine every meal. 虽然知道家人反对他喝酒，但他还是每餐喝一杯酒。
Though completed, the project was never released. 虽然项目已经完成，但却从未发布。
Although advertised as a green move, the proposal is riddled with contradictions. 尽管该提议被宣传为一项环保举措，但却充满了矛盾。
Although away on tour, Ariana Grande shared a video to thank her fans.
尽管正在巡演，爱莉安娜·格兰德还是分享了一段视频来感谢她的粉丝。
The infant, though at home, was under observation. 婴儿虽然在家，但仍在观察中.
The system, though well-meaning, is open to abuse. 尽管这个制度立意很好，却容易遭到滥用。
Although unable to speak because of the oxygen mask, he made it clear he was cold and wanted to come home. 虽然戴着氧气罩不能说话，但他明确表示自己很冷，想回家。
5）--as though，as if，as方式状语从句的省略
as if/though+doing/done/to do/介词短语/形容词
He stared at the girl as if seeing her for the first time. 他盯着那女孩，就像第一次见到她一样。
He feels powerless and helpless as if trapped in a nightmare. 他感到无力和无助，就像被困在噩梦中一样。
He shook his head as if to say: “Don’t trust her”. 他摇摇头，好像在说:“别相信她”。
I had a sensation of falling, as if in a dream. 我有一种坠落的感觉，像在梦中一样。
She left the room hurriedly as if angry. 她匆匆离开房间，好像生气了似的。
as+expected/required等，已成为惯用语
You should finish the homework as required. 你应该按照要求完成作业。
Engineers noticed that the pipes were not expanding as expected. 工程师们注意到管子并未像预期的那样膨胀。
The conference will be convened as scheduled. 会议将如期召开。
Everything went exactly as planned. 一切都严格地按计划进行。
The court of appeals gave that argument no weight, as described below. 上诉法院对这一论点不予重视，如下所述。(正如如下所述)
As mentioned above, information about your home is available at the local town hall. 如上所述，有关你家的信息可以在当地的市政厅获得。
6）--than/as...as...比较状语从句的省略
than+done/形容词，以及特有的省略形式
They sent us much more materials than required. 他们寄给我们的材料比所需要的多得多。
Progress was slower than expected. 进展比预计的缓慢。
We'll be stopping here for longer than scheduled. 我们在这里停留的时间将超出原先的安排。
This could result in spending more than desired. 这可能导致花费超出预期。
Use a kettle with an automatic cut-out so it doesn't boil for longer than necessary. 使用一个有断流器的水壶，这样水开后它会自动断电。
He wants a new bike more than his brother (does). 他比他哥哥更想要一辆新自行车。
He got more criticism than he deserved. 他受到了过多的批评。本质上省略了deserve后面的criticism。
as...as+done/形容词，类别as soon as possible
We'll get back to you as soon as possible. 我们将尽快再跟你联系。
I will trim my nose hair as often as necessary. 我会按需要经常修剪鼻毛。
The figure of seventy-million dollars was twice as big as expected. 7千万美元这个数字是预计的两倍。
Carbon prices will rise too slowly to cut industrial emissions as much as needed. 碳价格的上涨将过于缓慢，无法将工业排放削减到所需的水平。
We will stay as long as required. 需要待多久，我们就待多久。
We need as much money as they do. 我们和他们一样需要钱。do代替了need money.