状语从句同时具备下列两个条件: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。从句中的主语和be动词常可省略。例如：

    When ( the museum is ) completed , the museum will be open to the public next year .

    当博物馆完成，该博物馆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，他将去海边度假的话

    另外，比较状语从句经常省略。例如：

    I’m taller than he (is tall ).

    我比他高

    The higher the temperature (is), the greater the pressure (is ).

    温度越高，气压越大

    就状语从句而言,有时为了使语言言简意赅,常常将状语从句进行"简化"。状语从句的"简化"现象在口语中较为普遍,而且在高考中的复现率也较高。因此,有必要对其进行全面、透彻的了解。

    状语从句的"简化"现象常存在于以下五种状语从句中:

    ①由if, unless等引导的条件状语从句;

    ②由although, though, even if / though等引导的让步状语从句;

    ③由when, while, as, before, after, until / till等引导的时间状语从句;

    ④由as, as if等引导的方式状语从句;

    ⑤由as, than等引导的比较状语从句。下面针对这五种情形作一归纳。

    (1)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。

    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)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。常用于以下几种情形:

    连词+形容词

    As (he was) young, he learned how to ride a bike.

    他小时候就学会了骑自行车。

    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。

    连词+名词

    While (he was) a young boy, he was always ready to help others.

    他在孩子时代就乐于助人。

    Although (he was) a farmer, now he is a famous director.

    尽管他曾是个农民,但现在他是位著名的导演了。

    连词+现在分词

    As (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。

    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。

    连词+过去分词

    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。

    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。

    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。

    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 had mastered the English language before (he was) in the USA.

    他到美国之前就懂英语了。

    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。(=The meeting over）