be to do sth.和be about to do sth.有什么区别?

Question

如题所述

Answer 1

be to do sth约定,注定,必定要做什么.
如:They were to meet at the usual place.
他们约好在老地方见．
be about to do sth 正要做什么
如：He was about to leave when the phone rang.
他正要离开就在这时电话响了．

Answer 2

be to do sth.将要做某事，后面可以加时间状语
be about to do sth.即将做某事，常不加时间状语