问题描述：
there are mang boys playing the footballs
i 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?
i 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?
问题解答：
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