感官动词(see look 等)或使役动词(let make have)在变被动时都要在动词前加to,我很确定 所以:1 He was seen to play the piano by me yesterday. 2 He was seen to play the piano when i came in. 再就是:在任何情况下都没有seen to be doing这种形式,所以这样改的答案都错了
He was seen played the piano by me yesterday .这个需要强调 by me,因为不能确定是谁看到. He was seen playing the piano when I came in .这个不需要强调 by me,因为是我进来时看到的,所以不用太烦琐的强调.