冒号 就相当于 后面跟的内容，是对前面内容的完善
1、用在公函上收信人的称呼 == Dear Sir: （可变逗号 -- 下面是要说的话）
2 用于分开标题和副标题 (当一标题需要进一步说明时所加上的副标题)
Math Applications: Using Calculus to Determine the Age of Rocks
3、对句子加以补充，使其完善
a) 冒号后跟单词
A repairman must have three things: a screwdriver, a hammer, and a saw.
b)冒号后跟句子
The plan contains:
(1) .....
(2) ....
(3) ....
还有时间的分割也用冒号。比如 时间 7:30；比如 比例 2:1 等等
破折号 就是 后面跟的内容相当于前面内容的同位语， 作用如下：
1、表进一步解释
It's an environmental issue. —— That's not a small matter.
2、表插入
Jane ——one of my friends——came to see me yesterday.
3、表转折
"And may I ask—"said Pill，"but I thin it's better for you to ask him.
4、 表说明
Only one thing I have to do —— reading.