首先,你要知道,that 是關系代詞
where 是關系副詞
定語從句,目的是修飾名詞
如果名詞在從句中作主語,賓語等,那么就用關系代詞;作壯語,則用關系副詞
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 在從句中作地點狀語;后者作表語
通俗說就是把place 放到從句中,看是否構成句子,如果構成,則用that ;反之則用where
pretty good explanation