GMAT语法中有一个重要的考点，叫做平行。那么我们今天就来谈一谈平行。首先，什么是平行？平行可以近似理解为对称，或者说相似，或者用中文的修辞手法来说，有点像排比。平行首先追求的是形式上的一致。比如中文里，我们说“风花雪月”，这就是一个十分符合平行原则的词，“风”“花”“雪”“月”，四个名词，形式上非常一致；再如“龙飞凤舞”，这个词也非常平行，“龙飞”对应“凤舞”，形式非常对称和一致。这两个词，如果分别改一下，“风花雪月”改成“风花雪看月亮”，意思虽然也到了，但是读着就极其不舒服了，强迫症如我是不能忍的；同理，“龙飞凤舞”改成“龙飞舞凤”，也破坏了原来非常完美的平行，如果仅从平行的角度而言，改成“飞龙舞凤”，可能还是能接受的。
在GMAT语法中，在某些情况下，也希望句子的某些成分能实现平行，达成形式上的对称和一致。比如在出现以下两类词的时候，需要考虑平行：第一类词，我们把它们称为 closed markers (封闭性标志词)，如 not only…but also…\from…to...\both…and…等；第二类词，我们称之为 open markers (开放性标志词)，如 and\but\or 之类。这两类词出现时，首先要保证相应的平行成分，在形式上保持一致。
具体来说，比如在以下这个句子中，形式平行的原则贯彻的十分OK: I not only know him but also know well about him. “not only”和“but also”中间的成分为“know him”，是一个动词短语，but also后面的成分是“know well about him”，也是一个动词短语，形式上非常一致。而以下这个句子就不满足平行的原则：I not only know him but also I know well about him. 在这个句子中，“not only”和“but also”框出的平行成分是“know him”，一个介词短语，但是 but also 后面的是“I know well about him”，一个句子，所以两者形式上不平行。同理，在以下这两句中，句1：“She is not only beautiful but also kind.” 句2：“She is not only beautiful but also is kind.”前一句“beautiful”和“kind”实现了平行，而后一句“beautiful”和“is kind”则不平行。所以总结来说，在 close markers 中，成对出现的这两个词（如 not only 和 but also）把平行成分固定了下来，第二个词后面的成分要和该平行成分形式上保持一致。形式的描述如下：not only + 平行成分1 + but also+平行成分2，平行成分1、2形式上保持一致。
接下来我们可以看一道例题来进一步熟悉此点：
At the end of the 1930s, Duke Ellington was looking for a composer to assist him – someone not only who could arrange music for his successful big band, butmirroring his eccentric writing style as well in order to finish the many pieces he had started but never completed.
(A) someone not only who could arrange music for his successful big band, butmirroring his eccentric writing style as well in order to finish
(B) someone who could not only arrange music for his successful big band, but alsomirror his eccentric writing style in order to finish
(C) someone who not only could arrange music for his successful big band, but also to mirror his eccentric writing style in finishing
(D) that being someone who could not only arrange music for his successful big band, but mirroring his eccentric writing style
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