to be doing正在做某事 (其实就是现在进行时)
to have been doing过去正在做某事,现在还在做(通常表示延续)
当实际用的时候,to没有任何实际意义,所以这两个结构可以简写成:be doing,have been doing.
例:I'm singing(我在唱歌)
She has been playing the piano for 5 years.(她已经弹了5年钢琴).(她以前在弹,现在还准备弹下去).
不定式表将来是:going to do sth.
例:I'm going to have a good sleep.(我要好好睡一觉).
be doing也可以表将来,"进行时表将来.
例:She is flying to England in a week.(她一周后要飞往英国)