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十一、状语从句省略结构
Metals expand when they are heated. Metals expand 是主句;when they are heated 是状语从句。但是这个状语从句中可以省略掉they are, 句子变成：Metals expand when heated. 但是并不是所有的状语从句都可以这么省略。
这种省略从句主语的方式理论上需要满足以下 两个条件：
第一、从句主语和主句主语必须保持一致; 第二、从句的谓语必须是be动词，主语和be动词同进同出，比如上面的they和are要么同时省略，要么同时保留。
例1： Although___rigid, bones exhibit a degree of elasticity that enables the skeleton to withstand considerable impact. (A) apparently (B) are apparently (C) apparently their (D) are they apparently
分析：连词although通常引导主谓完整的状语从句，当从句主语与主句主语相同、从句谓语又为系表结构时，则可省略主语和连系动词，故正确答案为(A)。Although apparently rigid = Although bones are apparently rigid, 注意bones和are必须同时省略。
因此，我们从上面的例题中来分析一下填空题中状语从句省略以后的形式：though, although, even thought, while, If, when等为引导状语从句的词;这些词后面一定+形容词(分词)，主句(注意逗号的后面是主句，前面是从句)
例2： All marble is composed of crystals of the minerals calcite or dolomite, ___, are perfectly white.
(A) when, pure which (B) when, which pure (C) which, pure when (D) which, when pure
分析：空格前是个完整的句子，空格后是系表结构，空格处显然缺非限定性定语从句的引导词兼作从句主语。(A)、(B)不引导非限定性定语从句，可首先排除。(C)中的pure和when位置错了，故选(D)。注意when pure是插入语，同时也是省略用法when they are pure;which are perfectly white, 状语从句省略结构，
例3：___ relatively costly, the diesel engine is highly efficient and needs servicing infrequently
(A) Even (B) It is (C) Even though (D) There is
分析：B和D填入后，都是构成两个主句，不对;Even though则引导状语从句，even though it is, 这里it is 已经省略。