规则4 在引导独立子句的连词前加逗号[rule-4]

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.

由as（意为because）、for、or、nor或while（意为and at the same time）引导第二部分的两部分句子，同样需要在连词前加逗号。

如果一个从属子句，或一个需要用逗号隔开的引导性短语，位于第二个独立子句之前，那么连词后就不需要逗号。

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

当两个子句的主语相同且只出现一次时，如果连接词是but，逗号有用。当连接词是and时，如果两个陈述之间的关系紧密或直接，就应省略逗号。