i often hear this song_____(sing),but i have never hear her____(sing) it. 下面还有Listen!you can hear her____ it now.这三个空怎么填,讲一下为什么

i often hear this song_____(sing),but i have never hear her____(sing) it. 下面还有Listen!you can hear her____ it now.这三个空怎么填,讲一下为什么
i often hear this song_____(sing),but i have never hear her____(sing) it. 下面还有
Listen!you can hear her____ it now.这三个空怎么填,讲一下为什么

i often hear this song_____(sing),but i have never hear her____(sing) it. 下面还有Listen!you can hear her____ it now.这三个空怎么填,讲一下为什么
sung被动,sing主动,且表示频率,singing主动,且表示正在发生