It must be teenages having fun 这里为什么要用having 而不用to have

It must be teenages having fun 这里为什么要用having 而不用to have It Must Be?歌词 it must be It Must Be Love 歌词 It must be some catch. It must be him 歌词 it must be _____(interest) it must be rainy( )( )(随后) it must be imagination 歌词 It must be very happy. It must be something like it. 英语翻译it might be fun 和it must be there must be something wrong to it It must be there,---?反意疑问句 It must be she‘s 还是 hers? it must be great fun.great it' must be 后接什么 it must be love的中文意思