to be doing正在做某事 (其實(shí)就是現(xiàn)在進(jìn)行時)
to have been doing過去正在做某事,現(xiàn)在還在做(通常表示延續(xù))
當(dāng)實(shí)際用的時候,to沒有任何實(shí)際意義,所以這兩個結(jié)構(gòu)可以簡寫成:be doing,have been doing.
例:I'm singing(我在唱歌)
She has been playing the piano for 5 years.(她已經(jīng)彈了5年鋼琴).(她以前在彈,現(xiàn)在還準(zhǔn)備彈下去).
不定式表將來是:going to do sth.
例:I'm going to have a good sleep.(我要好好睡一覺).
be doing也可以表將來,"進(jìn)行時表將來.
例:She is flying to England in a week.(她一周后要飛往英國)