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1.当从句的主语和主句的主语一致(或者从句的主语为it)．
2.从句谓语中包含be动词．如：be doing,be done, be to do, be +adj
在此种情况下可以省去从句的主语和be动词，保留be之后的部分，被保留部分常为现在分词、过去分词、不定式、形容词等。
1. Don’t speak until（you are）spoken to.
2. I’ll buy a TV set if （it is ）necessary.
3. While（ I was ）walking walking alone in the street, I heard my name called.
4.Tom raised his hand as if (he was ) to say something.
1.when，while，as, until引导的时间状语从句的省略
While(they are)visiting the city,they reeeived a warm welcom
When(he was)asked why he was late，he kept silent．
2.if，unless，once引导的条件状语从句的省略
Onee(it is)begun,it must be done well．
Unless（you are invited) to speak, you should remain silent at the conference.
3.though，although,even if,even though引导的让步状语从句的省略
Though lacking money, his parents managed to send him to university.
4.as though，as if，as引导的方式状语从句的省略
He shook his head as if (he was）to say: “Don’t trust her”. She left the room hurriedly as if( she was) angry.
He stared at the girl as if (he was) seeing her for the first time.
You should finish the homework as (you are)required.
5. than引导的比较状语从句的省略
They sent us much more materials than required.
They had not met any more since leaving（=since they left）school.
Having finished（After he had finished）middle school，she entered Beijing University.
①原因状语从句如果主从句主语一致，也可以将从句省略为分词短语，介词短语或独立结构。如：
Being（=As he is）the oldest，Bill looks after his little sisters.
Because of smoking（=Because he smokes）too much，Linda doesn’t look healthy.
②so that，in order that引导的目的状语从句，若主从句主语一致，可以用动词不定式短语so as to或in order to代替。如：
I left early so as to（=so that I could）catch the first bus.
③so that，so…that引导的结果或程度状语从句如果主从句主语一致，可以用“enough + 不定式”或“too…to”结构代替。如：
Tom studied hard enough to pass（=Tom studied hard so that he passed）the examination.
She was too tired to（so tired that she couldn't）go any farther.
④if引导的条件状语从句，常可省去从句中的主语和系动词，转换为介词短语、不定式短语、分词短语或独立结构。如：
If（it is）possible，I will help you.
With her help（=If I had help from her），I could do the experiment.
⑤as，than引导的比较状语从句中，与主句相同的部分可以省略。如：
He is a head taller than I（am tall）.
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