It must be teenagers having fun 为什么这里要用having而不用to have

I think it must be teenagers having fun.这句话的句子结构是怎样的 it must be teenagers ___ fun.A.have B.having C.had D.to have It must be teenagers having fun 为什么这里要用having而不用to have but my friends and i think it must be teenagers having fun为什么用having Finally,teenagers must not be selfish.We should be kind to others_______.空里填什么吗?Finally,teenagers must not be selfish.We should be kind to others_______.空里填什么（it hard instead especial be） 可以变型 That must be teenagers having fun noise 语法错误吗 英语翻译以下如同1.he could be running for exercise.2.he might be running to catch a bus.3.it must be teenagers having fun 句子My friends and I think it must be teenagers having fun.中 为什么最后的have要用动词ING形式 it must be teenagers ------------fun.A have B having C had D to have选什么,为什么?有语法知识吗 Every night we hear strange noises outside I think it ____ be teenagers having fun ,though I'M not sureA.must Bshould C.will D.could It Must Be?歌词 it must be there are mang boys playing the footballsi think it must be teenagers having fun这当中的playing having 前为什么不用加be动词,这涉及到什么语法知识? It is reported that in New Mexico,not until they graduate_______toleave high school.A.will teenagers be allowed B.teenagers will be allowedC.will be allowed teenagers D.allowed teenagers will be It Must Be Love 歌词 It must be some catch. It must be him 歌词 it must be _____(interest)