现在进行时与过去进行时区别
现在进行时表示现在（当前），一般时间正在进行的动作。可以表示有计划的未来。构成形式：be+动词-ing.be是助动词，有人称和数的变化。即要根据主语选用am,is,are。过去进行时表示过去某时正在进行的动作（不强调是否完成）。构成为was、were+动词-ing.在其构成形式中be是助动词，同样有人称和数的变化，即要根据主语选用was、were。
表达将来时的几种结构:
01.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.
02.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)
