we will talk about the cases ----some rare animals become en

用where 或是that 主要看定语从句缺什么,比如这句话,从句不缺主语,宾语什么的,当然用不上单个的that,要用that 从句就得添加in 指“ 在...情况下” 而带in 的关系代词就等于where,
比较：
we will talk about the cases that he told us yesterday.
这一句之所以用that 是因为定语从句缺宾语.tell sb sth 缺个直接宾语