英語中為了表達同類思想觀點和行為動作而把兩個或兩個以上的意義相關、層次相同、句法功能也相同的詞、短語、句子等成串排列構成的結構序列稱之為平行結構(Parallelism)。這種結構在英語句子中的應用很廣泛，值得我們多留意。
當一句話裡面出現兩個或兩個以上的平行成分時，這些成分應該保持平衡，如果不注意這一點就會出現句子結構不平衡的問題，比如下面這幾個例子：
(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.
上面這幾個句子雖然語法上沒問題，但讀起來不夠順暢，這是因為句子結構不平衡。第一個句子to swim in the pool以及swimming in the river這兩個部分不一致，可以統一改為動名詞形式。第二個句子…and wrote songs too 應該和前面的名詞結構形成對應，可以改為…and a song writer。第三個句子warmth, in a humorous way這兩部分也要統一。第四個句子…but he knew little about his own country 要跟前面的who had lived in Brazil結構上保持一致。這些句子可以改為：
(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，修改後的句子結構更加緊湊，表現力也更強。同時，平行結構句與其他類型的句式結合能夠讓語言整散交錯、活潑生動，避免了單調、呆板，達到整齊勻稱又富有變化的形式美。很多英語名言都使用了平行結構，比如培根（Francis Bacon）有這樣一句話：
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.
作者使用一系列語法結構相同的平行句子來闡述他對讀書的見解——可淺嘗，可瀏覽，也可細嚼慢咽。
一般來說，英語平行結構有單詞平行、短語平行以及從句平行這幾種形式。
（1）單詞平行
由單詞組成的平行結構使用一組詞性相同、意義相關、語氣一致的詞來說明一個對象，這些單詞可能是名詞、動詞、形容詞、副詞、介詞等。比如邱吉爾有一句名言：
I have nothing to offer but blood, toil, tears and sweat.
blood, toil, tears, sweat這幾個名詞構成了並列成分，使得句子讀起來更有氣勢，更有感染力。
又比如歐巴馬就職演講中有這樣一句話：
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 這三個並列的單詞也組成了平行結構。
（2）短語平行
短語的平行結構包括名詞短語、動詞短語以及介詞短語等，比如下面這個例子：
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 共同結構是 the XX of… 它們構成了短語平行結構，使得句子更加有節奏感。
（3）從句平行
從句的平行並列包括各類從句的並置，例如賓語從句、定語從句和狀語從句等，比如同樣是歐巴馬就職演講中的句子：
…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從句的重複令人印象深刻，這種從句平行結構在演講中很常見，可以用於增強氣勢，調動觀眾情緒。
由此可見，恰當使用平行結構能大大地增加語言的表現力，使行文更流暢、結構更嚴謹、邏輯更嚴密。同時，熟悉平行結構也能幫助我們在閱讀中更好理解作者的語言風格，提高欣賞英語文學作品的水平。