She still remembers the days-she spent in the countryside.

因为定语从句中的动词spent后面缺少宾语.我们说spent （the days）.括号里是名词作宾语,所以用关系代词that 或者 which
如果改成...she lived in the countryside,那么就用where,因为lived后面不缺宾语,我们不能说lived the days,我们只能说lived (in the days),括号里是介宾短语作状语,不是宾语,所以用关系副词when.