1、用在公函上收信人的稱呼 == Dear Sir: （可變逗號 -- 下面是要說的話）
2 用於分開標題和副標題 (當壹標題需要進壹步說明時所加上的副標題)
Math Applications: Using Calculus to Determine the Age of Rocks
3、對句子加以補充，使其完善
a) 冒號後跟單詞
A repairman must have three things: a screwdriver, a hammer, and a saw.
b)冒號後跟句子
The plan contains:
(1) .....
(2) ....
(3) ....
還有時間的分割也用冒號。比如 時間 7:30；比如 比例 2:1 等等
破折號 就是 後面跟的內容相當於前面內容的同位語， 作用如下：
1、表進壹步解釋
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、表轉折
"And may I ask—"said Pill，"but I thin it's better for you to ask him.
4、 表說明
Only one thing I have to do —— reading.