关于定语从句的小问题~（错） This is the mountain village where I visited

关于定语从句的小问题~
（错） This is the mountain village where I visited last year.
（错） I will never forget the days when I spent in the countryside.
（对） This is the mountain village (which) I visited last year.
（对） I'll never forget the days (which) I spent in the countryside.
分析一下为什么错