这句话有没毛病The boys who are playing football are from Class One.

这句话有没毛病The boys who are playing football are from Class One.在踢足球的男孩们是一班的.

首先你明确一点,这句话包含定语从句,即who are playing football,这是定语,修饰boys,而后面的are是主句谓语,即The boys are from Class one
前面的are,是定语从句中的谓语,即The boys are playing football.
你也能这样结合两个句子,The boys who are from class one are playing football .句意相同
再问： 哦对哦！那我再请教你一个问题：I still remember the place where I met her for the first time. 中的where可以换成that吗？
再答： 不可。可换为in which 按照我们老师讲解，这样理定语从句部分是where I met her for the first time,'where‘是关系副词，代指in the place.在定语从句中作地点状语，如此你可认为定语部分是I met her for the first time in the place。in which更好理解，in which中which代指the place
再问： where为什么不能换成that？？that不也可以可以作定语从句的连接词吗，而且换在这里也讲得通呀.
再答： 你问的好快，我还没打完那，汗。。。 我仔细解释一遍，关系词一共两种，关系代词who,which.whose.that（还有个不常提的as）,关系副词where,when,why。根据先行词在从句中所作成份选择关系词。作状语，选择where when why。句中the place在从句中作状语，即作“我第一次遇到她”的状语。又可比较when的用法：I still remember the day when(on which) I first met you 所以定语部分是I met her for the first time in the place。主句是I still remember the place.两句一结合，就成了你说的那个句子。补充。。你以后可多找一些定语从句拆成两分句，就会发现关系代词，副词都是怎么选择的了。我可以给你举几个复杂点的例子 The teacher told students to hand in their paper,all of whom are still busy thinking of problems 拆开The teacher...paper.All of them are...problems.这里的whom代指srudens，又因为前面有all of（of是介词）故用whom不用who I stand before the window ,through which I could find some sheep。你可以拆开试试
再问： 好杂啊，是不是作状语就不能用that了？
再答： 是的，如果先行词在从句中作宾语，主语，就可用which或that（先行词是物时）