sb be to do 与 sb be about to do 的区别

be to do一般是表示按计划进行或征求对方意见
e.g.I am to go to school tomorrow.
Are we to go on with this work?

be about to 即将发生的动作，指眼皮底下就要发生的事情（对了，一般是不跟时间状语的）
e.g.Look,the train is about to leave!

固定结构：
be about to do sth when...当某事发生时，正好...
He was just about to stand up when the teacher came in.
老师进来的时候，他刚要站起来。

http://zhidao.baidu.com/question/16442920.html?si=3
http://zhidao.baidu.com/question/18357468.html?si=1

I am to watch a movie tonight.=be going to
be about to do马上要做什么事