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？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 placewhere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.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whoI stand before the window ,throughwhich I could find some sheep。你可以拆开试试好杂啊，是不是作状语就不能用that了？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）