We often see him sing in the classroom为什么sing不用加s啊?

We often see him sing in the classroom为什么sing不用加s啊? We often sing in the classroom.(sing划线） ____________________ We often hear her sing in the next room中为什么用sing而不用sings We often hear him ______ in the classroom.A.sing B.singing C 改被动:We often hear them sing in the classroom. 分析句子成分：We often hear him sing in the hospital. Listen Joan (sing)in the classroom .She often (sing)there. I often hear him _____ ( sing ) in the next room.为什么答案上要求加in we often sing in the music club.sing划线提问 I heard him sing in the classroom改为被动语态? He often sees her sing in the classroom有错么?为什么? We often see her dance in the room .为什么第三人称单数her后dance不用加s?