He often plays football.他经常踢足球.
He was playing ball when his friends came.他朋友来时他正在踢足球.
描述一件正在进行（或某个时刻正在进行的事情）时,用进行时.
描述一件按一定频率（周期）发生的事情时用三单.He plays ball
He playing ball为什么不一样没有He playing ball,只有He is/was playing ball.
He plays ball描述经常发生的动作

He is playing ball描述某个时刻正在进行的动作