be to do和be doing有什么不同呢？

be to do 表示将来,一般是按计划安排或者注定、应该要做的事情.
例如：
We are to attend the meeting at three p.m.
我们将在下午三点参加会议。
You are to get here at eight in the morning.
你应该在明天早上八点到这里。
be doing 是现在进行时表将来 现在进行时表示将来意义时使用的动词通常为瞬间动词,如：come,go,arrive,leave,start,return,meet,get等。
例如：
They are leaving for Shanghai tomorrow.
他们明天将离开前往上海。

祝你开心如意！

扩展阅读：be born to do ... be here to do ... be abundant to do ... be going to do ... be free to do ... be tough to do ... be said to do ... buy up ... seem to do ...