Question

I will never forget the days whiich I spent in the countryside last year.为什么不用when而用which?

Answer 1

which关系代词，定语从句中 缺乏主语、宾语时使用
这里 I spent the days spent缺了宾语 “度过那些日子”
where 关系副词，在成分基本完整时，选用
如: I will never forget the days which i lived in the countryside .
如有疑问，欢迎追问。

Answer 2

因为 spent 是及物动词 , which作spent的宾语
which引导的是定语从句

Answer 3

which在这里替代the days，the days是用在spent的后面的宾语，所以用which

Answer 4

我补充一下，spend是及物动词，必须要带有宾语。如果要用when的话，就可以说The days when i stay in the…因为stay是不及物动词。