are being built是什么时态

　　表达将来时的几种结构:

　　be going to do：

　　表示最近打算进行的事,具有一定的目的性和计划性.(这种安排既有可能是主语做出的,也有可能是别人安排 的)

　　We are going to have a meeting tomorrow.

　　表示即将要发生的事情，不可避免要发生的事情.(多用于表天气)

　　It's getting cloudy. It is going to rain.

　　be to do：

　　表示按计划要发生的事,一般认为,这种计划是双方约定的,而 be going to do 所指的安排不一定是双方约定 的.

　　We are going to meet them at the north of the bridge, because it is the only road they can follow. 我们打算去桥的北边堵他们,因为那是他们的必经之路.

　　We are to meet each other with them at the north end of the bridge. 双方约定好要在桥的北边见面.

　　be to do 还有很多情态用法,be going to do 没有以下用法.

　　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.(表责任,义务,相当于,must should) Are we to clean the corridor,Mrs Dell? (表征求意见=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?(表假设)