表達將來時的幾種結構：
be going to do：
表示最近打算進行的事,具有一定的目的性和計劃性.(這種安排既有可能是主語做出的,也有可能是別人安排 的)
We are going to have a meeting tomorrow.
表示即將要發生的事情，不可避免要發生的事情.(多用于表天氣)
It's getting cloudy. It is going to rain.
be to do：
表示按計劃要發生的事,一般認為,這種計劃是雙方約定的,而 be going to do 所指的安排不一定是雙方約定 的.
We are going to meet them at the north of the bridge, because it is the only road they can follow. 我們打算去橋的北邊堵他們,因為那是他們的必經之路.
We are to meet each other with them at the north end of the bridge. 雙方約定好要在橋的北邊見面.
be to do 還有很多情態用法,be going to do 沒有以下用法.
You are to wash your hands before every meal.(表需要=need)
As Warrent's parents,you are to set a good example to him.(表責任,義務,相當于,must should) Are we to clean the corridor,Mrs Dell? (表征求意見=shall)
The temperature is to drop soon.(表示可能性)
Would she get cross if I were to sail into her of lying?(表假設)