狀語從句省略結(jié)構(gòu)
Metals expand when they are heated. Metals expand 是主句;when they are heated 是狀語從句。但是這個狀語從句中可以省略掉they are, 句子變成：Metals expand when heated. 但是并不是所有的狀語從句都可以這么省略。
這種省略從句主語的方式理論上需要滿足以下兩個條件：
第一、從句主語和主句主語必須保持一致;
第二、從句的謂語必須是be動詞，主語和be動詞同進同出，比如上面的they和are要么同時省略，要么同時保留。
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：連詞although通常引導(dǎo)主謂完整的狀語從句，當(dāng)從句主語與主句主語相同、從句謂語又為系表結(jié)構(gòu)時，則可省略主語和連系動詞，故正確答案為(A)。Although apparently rigid = Although bones are apparently rigid, 注意bones和are必須同時省略。
因此，我們從上面的例題中來分析一下填空題中狀語從句省略以后的形式：though, although, even thought, while, If, when等為引導(dǎo)狀語從句的詞;這些詞后面一定+形容詞(分詞)，主句(注意逗號的后面是主句，前面是從句)
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：空格前是個完整的句子，空格后是系表結(jié)構(gòu)，空格處顯然缺非限定性定語從句的引導(dǎo)詞兼作從句主語。(A)、(B)不引導(dǎo)非限定性定語從句，可首先排除。(C)中的pure和when位置錯了，故選(D)。注意when pure是插入語，同時也是省略用法when they are pure;which are perfectly white, 狀語從句省略結(jié)構(gòu)，
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，都是構(gòu)成兩個主句，不對;Even though則引導(dǎo)狀語從句，even though it is, 這里it is 已經(jīng)省略。