有人听说她昨晚在她房间里弹钢琴.
这句话用主动语态表述就是：
Someone heard her play the piano in her room last night.
hear sb do sth （听到某人做某事）这个结构中,hear是感官动词,后面的宾语补足语要用 原形,尽管它是过去的动作.这种结构的句子变为被动语态后,宾补就不能再用动词原形了,要加上to：
She was heard to play the piano in her room last night.