在阅读和训练时，常常能遇见诸如when/while+doing/done/adj./介词短语等类似结构，要灵活把握这类句型的含义，如果能在作文中熟练而巧妙地运用一句，作文档次无疑会提高一个档次，是什么概念考生自己去想。这其实就是状语从句中的省略知识，请看：
省略的原则：
1) 从句主语与主句主语一致，且谓语是be动词，则，主语+be一起省略。例如：
Be careful when (you are) crossing the street.
过马路时要小心。
He wrote his greatest novel while (he was) working on a freighter.
当他在货船上工作的时候，他写出了他最伟大的小说。
也会出现省略的一种转换：状语从句中，主从句的主语一致时，可以在从句中省略主语，其谓语动词采用doing/done,的形式：
When you drive a car, you should be alert.
When driving a car, you should be alert.
当你开车时，你应该提高警惕。
其实这也是下面的第三个原则。
2) 从句主语是it, 且谓语是be动词，则主语+be一起省略。例如：
If (it is) necessary, you can contact me at home.
必要的话，我在家时你也可以和我联系。
If (it is) necessary I can come at once.
如果有必要，我可以马上来。
If (it is) possible, you want to avoid alcohol. 你应尽可能避免饮酒。《牛3) 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。
4）when doing/while doing已经成固定用法，不必拘泥于主语是否一致，例如：
When eating a whole cooked fish, you should never turn it over to get at the flesh on the other side...
吃一整条鱼时，切忌把它翻过来去吃另一面的肉。
When travelling on fast roads at night it is impossible to drive and navigate at the same time.
晚上在高速路上行驶时根本做不到边开车边找路。
Terry actually says 'Oh, shucks!' when complimented on her singing.
人们赞誉她的歌声时，特里实际上会说“噢，没什么！”
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。
5)省略多用于写作中，起到精炼及形式变化的作用，但省略并不是必须的，偶尔用用即可获得意想不到的的妙处。
分类说明：
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 doubt, call the doctor.
如有疑问，请联系医生。
When parking on a hill, leave the car in gear.
在斜坡上停车时把汽车挂上挡。
When (you are) in Rome, do as the Romans do. 入乡随俗。
When at home, most evenings would be spent reading, cutting up and filing away his photographs.
晚上在家时，经常都是在看书或剪切和归类他的照片。
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。
She got married while working as a waitress.
她在饭店当服务生期间结婚了。
Pat killed time by watching TV while waiting for her.
帕特在等她时边看电视边打发时间。
It was alleged that he had taken bribes while in office.
他被指称在任时收受贿赂。
He also talked briefly about the isolation he endured while in captivity.
他还简单谈及了他被俘期间忍受的孤独。
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。
The submarine is invulnerable to attack while at sea.
潜艇在海上是不会受到攻击的。
While at the beach this afternoon, I met a boy called Bob.
今天下午我在沙滩上遇到了一个叫鲍勃的男孩。
中考真题- 2018 天津
They met while on holiday in Greece.
他们是在希腊度假时认识的。
She vanished nine months ago while on a mercy mission to West Africa
9个月前她在去西非的慈善之旅中消失了。
He was taken hostage while on his first foreign assignment as a television journalist.
他第一次作为电视记者出国采访时就被扣作人质。
He died suddenly while on a skiing holiday with his family in Val d'Isere.
他与家人在瓦勒迪泽尔滑雪度假时猝然过世。
The workers held a banner reading 'We Want Bread' while on strike.
工人们罢工时打着横幅，上面写着“我们要面包”。
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。
由He wants a new bike more than his brother wants a new bike 省略而来
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.