I heard them sing in the classroom.是对的,为什么I heard them singing in the classroom.不对?正在唱歌不应该用singing吗?

I heard them sing in the classroom.是对的,为什么I heard them singing in the classroom.不对?正在唱歌不应该用singing吗? I often heard () (sing) in the room we heard them sing in shanghai 改为被动语态 I heard someone sing in the next room. 为什么用 sing last sunday we heard them sing peking opra in that theatre改为被动语态 Last Sunday we heard them sing Peking Opera in that theater.应改为They were heard to sing Peking OLast Sunday we heard them sing Peking Opera in that theater.应改为They were heard to sing Peking Opera in that theater last Sunday.为什么不可 I heard her ＿（sing） in the next room when I came back. I heard him sing in the classroom改为被动语态?someone say it should be modified likehe was heard to sing in the classroom.is that correct? I heard someone sing in the next room last night.语法有误吗?I heard someone sing in the next room last nigh.sing要不要改成sang?为什么?这题是sing没错.不用改成singing. I heard her___ in the next room just now.A sing B singing I heard the girl---（ sing)in the nest room yesterday evening. I heard the gril ___ in the room.A.singing B.sing I heard the girl ___(sing) in her room at this time last night. 一个初一语法首先看一下例句：1、I often hear them sing songs in the next room.2、I saw you put the key in your pocket.3、I hear them singing songs in the next room.4、I heard her singing in the classroom.第一问（1和2):难道差 .Yesterday,Ben and Kitty __________ (be) at home.They __________ (sing) a lot of songs.I heard them _____________ (sing) loudly. Mary ___ some songs in her room in the morning.A.heard sing B.heard to sing C.was heard sing D.is heard to sing为什么? I heard him sing a song in the next room just nom .(改为同义句）He ( ) ( )( ) sing a song in the next room just nom. People in the city kept their sorroundings _______(protect).They don't allow unknown people _______(invite) to the ball.I found a strange person _______(walk)nearby our shop yesterday.We heard the song _______(sing)next door.We heard them _______