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.
差不多。
be about to 有即将做某事的意思
be to do 是将要做,但并没有上面那么急的意思
be to do 是将要做~与说话时间间隔不定~
be about to do是马上要做~正要做~
be to do 是将来
be about to do 是马上做,只能用于现在
be todo和BE ABOUT TODO都是将来要做某事的意思,用于将来时.但BE TODO 只表示将来,没有具体时间限制.BE ABOUT TODO更接近于"马上,即将做..."