定语从句的关系词不光只有that的
关从的关系词分为：关系代词和关系副词
关系代词有：that/which/whose/whom/who
关系副词有：when/where/why
有时候副词可以用“介词+关系代词”来表示.
比如说：This is the school in which I studied.
=This is the school where I studied.
这里的in which就相当于where
定从其实你可以拆成两个句子.这样就清楚了：
1.This is the school
2.I studied in the school.
the school用which来代替,那你看一下根据第二句是不是还有个in啊,这就是为什么会有介词的原因.