1) 解释说明，以插入语的形式解释前面提到的概念(此时破折号要前后对称)

　　e.g. A glance at a map of the Pacific Ocean reveals that there are many islands far out at sea that are actually volcanoes----many no longer active, some overgrown with coral----that originated from activity at points in the interior of the Pacific Plate.

　　如果破折号后面的解释说明出现在整句的最后了，那么只需要前破折号即可。

　　e.g. A glance at a map of the Pacific Ocean reveals that there are many islands far out at sea that are actually volcanoes----many no longer active, some overgrown with coral.

2) 强调插入的句子成分

　　The only person that he admires is --Steven.

3)突然转折，表转折意思的破折号常和but等转折词一起连用but--

　　here comes to逗号(comma) :逗号不能直接连接两个独立的分句。两个独立的句子可用句号分开。

　　NB .当出现x,y,and z 和 x,y and z时，我们倾向于选择前者。

4）两个逗号用来分开插入语，同位语，主从句等

　　a. Tom, as we all know, is a prodigy.

　　b. We have eliminated all statistical tables, which are of interest to the specialist。

　　c. Tom，a prodigy in math, won a special distinction from the king.

5）三者以上并列(列举或者形容词并列等)

6）常见于逻辑主语中 Hearing the news, I feel very sad. NB.主语直接跟谓语时，主谓之间不加逗号隔开。