首先,你要知道,that 是关系代词
where 是关系副词
定语从句,目的是修饰名词
如果名词在从句中作主语,宾语等,那么就用关系代词;作壮语,则用关系副词
for example,
1.I can't forget the place where I was born.
2.I can't forget the place that I was born in.
前者的place 在从句中作地点状语;后者作表语
通俗说就是把place 放到从句中,看是否构成句子,如果构成,则用that ;反之则用where
pretty good explanation