be to do表示一种命中注定会发生的事情
will do 表示一种个人意愿（将会做某事,这个主要和be going to do区分）
be about to do表示即将、马上、眼瞅着要做某事
例句：
1.Two boys were saying goodbye to each other without knowing they were not to meet again.
2.I'll go to London.
3.The telephone rang when I was about to go out.