The Elements of Style
Elementary Rule 4. Place a comma before a conjunction introducing an independent clause
e.g. The early records of the city have disappeared, and the story of its first years can no longer be reconstructed.
When the subject is the same for both clauses and is expressed only once, a comma is useful if the connective is but. When the connective is and, the comma should be omitted if the relation between the two statements is close or immediate. （当两个句子具有相同的主语，如果以but连接，用逗号分隔；如果以and连接，则无需逗号）
e.g. I have heard the arguments, but am still unconvinced.
He has had several years' experience and is thoroughly competent
Elementary Rule 5. Do not join independent clauses with a comma.
If two or more clauses grammatically complete and not joined by a conjunction are to form a single compound sentence, the proper mark of punctuation is a semicolon. (用分号分隔两个缺乏连词连接的相关性从句)
e.g. Mary Shelley's works are entertaining; they are full of engaging ideas.
也可以用句号（periods）连接两个独立的句子。
Mary Shelley's works are entertaining. They are full of engaging ideas.
如果存在连词，则用逗号连接两个句子：
Mary Shelley's works are entertaining, for they are full of engaging ideas.
如果是以副词连接两个句子，仍然使用分号：
I had never been in the place before; besides, it was dark as a tomb.
An exception to the semicolon rule is worth noting here. A comma is preferable when the clauses are very short and alike in form, or when the tone of the sentence is easy and conversational. （当两个句子非常简短，形式相似，或者隐含着相互的对话，则可以用逗号连接。意味着两个句子彼此非独立）
e.g. Man proposes, God disposes.
The gates swung apart, the bridge fell, the portcullis was drawn up.
I hardly knew him, he was so changed.
Here today, gone tomorrow.
