甚麼時候用「will」；甚麼時候用「be going to」？

USE 1 "Will" to Express a Voluntary Action
"Will" often suggests that a speaker will do something voluntarily. A voluntary action is one the speaker offers to do for someone else. Often, we use "will" to respond to someone else's complaint or request for help. We also use "will" when we request that someone help us or volunteer to do something for us. Similarly, we use "will not" or "won't" when we refuse to voluntarily do something.

Examples:

I will send you the information when I get it.
I will translate the email, so Mr. Smith can read it.

USE 2 "Will" to Express a Promise
"Will" is usually used in promises.

Examples:

I will call you when I arrive.

USE 3 "Be going to" to Express a Plan
"Be going to" expresses that something is a plan. It expresses the idea that a person intends to do something in the future. It does not matter whether the plan is realistic or not.

Examples:

He is going to spend his vacation in Hawaii.

Hope it will help you.

其實be going to和will都可以用來說到未來的事情，但be going to比will就有早有安排的意思；而will著重”意決”
所以，若做練習，說到事情已作好安排，就用be going to
但若強調你的意願或沒有甚麼特別安排就用：will

2007-12-03 20:55:52 補充：
詳情可參考這篇文章http://education.atnext.com/index.cfm?fuseaction=Article.View&articleID=6148922&issueID=20060721

兩個也是將來式。
用「will」是打箅會去做; 用「be going to」就肯定 會去做。

舉例
e.g. I will see the movie. (未太肯定會去)
e.g. I am going to see the movie. (已經肯定會去)