he will have been playing on the piano by that time 为什么要用on the?去掉会怎么样呢?意思是他将一直弹钢琴到那个时候。
问题描述：
he will have been playing on the piano by that time 为什么要用on the?去掉会怎么样呢?
意思是他将一直弹钢琴到那个时候。
答