以連詞and或but引導完整句子，連詞前方要加逗號

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以連詞and或but引導完整句子，連詞前方要加逗號

• The early records of the city have disappeared, and the story of its first years can no longer be reconstructed.
• The situation is perilous, but there is still one chance of escape.

這種類型的句子獨立於其上下文，雖然用連詞進行了連接，但可以視情況來變換句型。因為在它們中逗號之前是個完整的句子，連詞之後的句字算是對前面句子的補充說明，兩句話間的連接關係並不緊密。此外，在獨立的句子間使用and僅表示兩個句子之間存在某種關係，而未闡述具體關係。在上面的例子中，這兩個句子也可以寫成：

• As the early records of the city have disappeared, the story of its first years can no longer be reconstructed.
• Although the situation is perilous, there is still one chance of escape.

也可以用短語來替換從句：

• Owing to the disappearance of the early records of the city, the story of its first years can no longer be reconstructed.
• In this perilous situation, there is still one chance of escape.

對於由兩部分構成的複合句而言，要在as（表示「因為」）、for、or、nor和while（表示「同時」）所引導的第二個句子的連接詞前加上逗號。

如果一個從句或者一個需要被逗號隔開的插入語位於第二個主句之前，在連接詞之後就不需要加逗號了。

• The situation is perilous, but if we are prepared to act promptly, there is still one chance of escape.

如果一個主語後跟隨兩個分句，當連詞是but時，連詞前往往要加逗號；當連詞是and時，連詞前一般不需要加逗號。

• I have heard the arguments, but am still unconvinced.
• He has had several years experience and is thoroughly competent.

來源：The Elements of Style