This is the school.I studied in this school five years ago.

This is the school (in which)where I studied five years ago。先找主句（一般句型较简单）,this is the school,然后确定先行词（主句与从句重叠的词应该就是）,接着确定先行词在从句中充当的成分,最后根据充当的成分选择关联词将句子链接。