be to, be going to与be about to的区别

1）be to:
（1）表示事先商定、安排或准备要做的事情。如：
①The students are to meet at the school gate tomorrow．明天学生们将在学校大门口集合。
②Mike and I are to go skating after school this afternoon．我和迈克打算今天下午放学后去滑冰。
（2）表示可能性，必要、责任、义务、禁止等如：
①Her necklace was not to (couldn't) be found．她的项链找不到了。
②Our exercise-books are to (=must) be handed in before school is over． 我们的作业必须在放学前交。
③Teachers are to （=should或ought to）be respected by the whole society． 教师应当受到全社会的尊重。
④People are not to (mustn't) smoke in the office．办公室里禁止吸烟。

2）be going to:
（1）表示将要发生的事情或打算最近要进行的动作。如：
①It is going to rain soon． 快要下雨了。
②We are going to help the farmers with their autumn harvest next week．我们打算下周去帮助农民秋收。
（2）在含有条件状语从句的主从复合句中，主句一般不用be going to，而常用will（第一人称用shall）。如：
The football match will be put off if it rains tomorrow．如果明天下雨，足球赛将被推迟举行。

3）be about to:
（1）表示即将发生的动作，在时间上指最近的将来。如：
We are about to start． 我们就要出发了。
The new school year is about to begin． 新学年开学在即。
（2）在含有be about to的句子中，不能再加时间状语。如：
Wrong: The medical team is about to start immediately．
Right: The medical team is about to start． 医疗队就要出发了