be to do sth,be about to do sth,be going to do sth．

be to do sth．表示按计划或安排即将发生的动作，后可跟时间状语。如： 
You're to hand in your papers by 10 o'clock．十点钟以前你得交上试卷。

be about to do sth．表示打算或安排即将发生的动作，它通常不与时间状语连用。如： 
I was about to go out when someone knocked at the door．我正要出去，这时有人敲门。 

be going to do sth．有三层含义： 
①表示打算、计划或决定要做某事。如： 
We're going to spend our holidays in Wales this year． 今年我们打算到威尔士度假。 
②用以表示某事物即将发生或很可能发生。如： 
I'm going to be twenty next month． 下个月我就二十岁了。 
③有迹象表明即将发生的动作或状态。如： 
Look at those black clouds, there is going to be a storm． 瞧那些乌云，暴风雨就要来了。