首先你明確一點,這句話包含定語從句,即who are playing football,這是定語,修飾boys,而后面的are是主句謂語,即The boys are from Class one
前面的are,是定語從句中的謂語,即The boys are playing football.
你也能這樣結(jié)合兩個句子,The boys who are from class one are playing football .句意相同哦對哦！那我再請教你一個問題：I still remember the place where I met her for the first time. 中的where可以換成that嗎？不可。可換為in which按照我們老師講解，這樣理定語從句部分是where I met her for the first time,'where‘是關(guān)系副詞，代指in the place.在定語從句中作地點狀語，如此你可認(rèn)為定語部分是I met her for the first time in the place。in which更好理解，in which中which代指the placewhere為什么不能換成that？？that不也可以可以作定語從句的連接詞嗎，而且換在這里也講得通呀.你問的好快，我還沒打完那，汗。。。我仔細(xì)解釋一遍，關(guān)系詞一共兩種，關(guān)系代詞who,which.whose.that（還有個不常提的as）,關(guān)系副詞where,when,why。根據(jù)先行詞在從句中所作成份選擇關(guān)系詞。作狀語，選擇where when why。句中the place在從句中作狀語，即作“我第一次遇到她”的狀語。又可比較when的用法：I still remember the day when(on which) I first met you 所以定語部分是I met her for the first time in the place。主句是I still remember the place.兩句一結(jié)合，就成了你說的那個句子。補充。。你以后可多找一些定語從句拆成兩分句，就會發(fā)現(xiàn)關(guān)系代詞，副詞都是怎么選擇的了。我可以給你舉幾個復(fù)雜點的例子The teacher told students to hand in their paper,all of whom are still busy thinking of problems拆開The teacher...paper.All of them are...problems.這里的whom代指srudens，又因為前面有all of（of是介詞）故用whom不用whoI stand before the window ,throughwhich I could find some sheep。你可以拆開試試好雜啊，是不是作狀語就不能用that了？是的，如果先行詞在從句中作賓語，主語，就可用which或that（先行詞是物時）