i often hear the sound of children (playing) in the park.请解释为啥用playing?为啥用playing,为啥不用to play或者其他答案,是何种用法?并请再举点相似例子

I often hear the little girl___(cry)in the morning I often hear him _____ ( sing ) in the next room. I often hear the song _____on TV.（sing） I often hear him ( ) in the room I often heard him ( ) in the room I often hear the sound of the violin______（come） from the park nearby I often hear him sing the song in the room.变为被动语态 The sou什么意思? I often hear him sing the same song被动语态He ____ often ____ ____ sing the same song. I often hear her _____ (sing)in the next room last year.为什么不能用singing? I often hear her _(sing) in the morning ,填什么?为什么 I often hear him（）in the next room.（sing） I often hear birds ( )in the morning?是填to sing 还是填singi I often ____(listen to,hear) her reading in the room.急 I often hear him _____ ( sing ) in the next room.为什么答案上要求加ing? I often hear birds____in the tree.A.sing B.C.sings D./ I often hear the girl sing beautiful songs 改为被动语态怎么改 把I often hear him play the piano all day 改为被动语态 I often hear him ( )the pop songs .A sing B sings C singing D to sing