1)be going to do结构主要表示"意图"(即打算在最近的将来进行某事)和"预见"(即现在已有迹象表明将要发生或即将发生某种情况)。 【例如】 we are going to put up a building here. how are you going to spend your holiday?there is going to be a lot of trouble about this.there is going to be a meeting here this evening.that is what i am going to say.2)be to do结构通常表示正式的计划、指示、命令、禁止等或用来征求对方的意见。 【例如】when is the factory to gosintosproduction?the line is to be opened to traffic next week.when are they to hand in their plan?tom is to study medicine as soon as he has finished the military.you are not to tell her anything about our plan.3)to be about to do/be on the point of doing结构表示"将要，正要做……"，强调将来要发生的事情已经临近。 【例如】 the plane is about to take off.he was about to say something more when his wife came in.i can t see you now. i am on the point of leaving.