be to do是指将要作,但没有说什么时候作;be about to do是指马上作,立即作
两者区别在于:
be to do:有责任,义务,约定的含义.
e.g Parents are to take care of their children.
be about to do:表即将做某事.
e.g The plane is about to take off.
to do 是将要去做而还没有做的事。
about to do是即刻就要做的事。如我刚要打电话,就听到有人敲门。见下面解释和例句:
about means Ready to, on the verge of.例句: I was about to leave when it began to rain, or He hasn't finished yet but he's about to.
be about to do意思是即将做什么
前者只不过陈述事实而已