英语中为了表达同类思想观点和行为动作而把两个或两个以上的意义相关、层次相同、句法功能也相同的词、短语、句子等成串排列构成的结构序列称之为平行结构(Parallelism)。这种结构在英语句子中的应用很广泛，值得我们多留意。

当一句话里面出现两个或两个以上的平行成分时，这些成分应该保持平衡，如果不注意这一点就会出现句子结构不平衡的问题，比如下面这几个例子：

(1) To swim in the pool is not as fun as swimming in the river.

(2) Jimmy Davis was an amateur country music singer, a governor of Louisiana, and wrote songs too.

(3) He spoke with warmth and in a humorous way.

(4) We met a Frenchman who had lived in Brazil, but he knew little about his own country. 

上面这几个句子虽然语法上没问题，但读起来不够顺畅，这是因为句子结构不平衡。第一个句子to swim in the pool以及swimming in the river这两个部分不一致，可以统一改为动名词形式。第二个句子…and wrote songs too 应该和前面的名词结构形成对应，可以改为…and a song writer。第三个句子warmth, in a humorous way这两部分也要统一。第四个句子…but he knew little about his own country 要跟前面的who had lived in Brazil结构上保持一致。这些句子可以改为：

(1) Swimming in the pool is not as fun as swimming in the river.

(2) Jimmy Davis was an amateur country music singer, a governor of Louisiana, and a song writer.

(3) He spoke with warmth and humor.

(4) We met a Frenchman who had lived in Brazil but who knew little about his own country. 

可以看到，修改后的句子结构更加紧凑，表现力也更强。同时，平行结构句与其他类型的句式结合能够让语言整散交错、活泼生动，避免了单调、呆板，达到整齐匀称又富有变化的形式美。很多英语名言都使用了平行结构，比如培根（Francis Bacon）有这样一句话：

Some books are to be tasted, others to be swallowed, and some few to be chewed and digested; that is, some books are to be read only in parts; others to be read, but not curiously; and some few are to be read wholly, and with diligence and attention.

作者使用一系列语法结构相同的平行句子来阐述他对读书的见解——可浅尝，可浏览，也可细嚼慢咽。

一般来说，英语平行结构有单词平行、短语平行以及从句平行这几种形式。

（1）单词平行

由单词组成的平行结构使用一组词性相同、意义相关、语气一致的词来说明一个对象，这些单词可能是名词、动词、形容词、副词、介词等。比如丘吉尔有一句名言：

I have nothing to offer but blood, toil, tears and sweat.

blood, toil, tears, sweat这几个名词构成了并列成分，使得句子读起来更有气势，更有感染力。

又比如奥巴马就职演讲中有这样一句话：

Time and again, these men and women struggled and sacrificed and worked till their hands were raw so that we might live a better life.

句子中struggled, sacrificed, worked 这三个并列的单词也组成了平行结构。

（2）短语平行

短语的平行结构包括名词短语、动词短语以及介词短语等，比如下面这个例子：

They understood that our power alone cannot protect us, nor does it entitle us to do as we please. Instead, they knew that our power grows through its prudent use. Our security emanates from the justness of our cause; the force of our example; the tempering qualities of humility and restraint.

句子中加粗部分the justness of our cause, the force of our example, the tempering qualities of humility and restraint 共同结构是 the XX of… 它们构成了短语平行结构，使得句子更加有节奏感。

（3）从句平行

从句的平行并列包括各类从句的并置，例如宾语从句、定语从句和状语从句等，比如同样是奥巴马就职演讲中的句子：

…we cannot help but believe that the old hatreds shall someday pass; that the lines of tribe shall soon dissolve; that as the world grows smaller, our common humanity shall reveal itself; and that America must play its role in ushering in a new era of peace.

句子中一系列that从句的重复令人印象深刻，这种从句平行结构在演讲中很常见，可以用于增强气势，调动观众情绪。

由此可见，恰当使用平行结构能大大地增加语言的表现力，使行文更流畅、结构更严谨、逻辑更严密。同时，熟悉平行结构也能帮助我们在阅读中更好理解作者的语言风格，提高欣赏英语文学作品的水平。