用that
when 引導(dǎo)的是時間狀語從句,而本句是定語從句.
將定語從句單獨抽出來,完整的句子應(yīng)該是：I spent （the days) in the country.
這里的that指代the days,是定語從句中的賓語.
如果用when,從句中應(yīng)該不缺少任何成份,用when加了個時間狀語而已.而本題中的從句中少了一個賓語,所以,應(yīng)該用that.
如果將spent改成was,就用when做連詞了.
I'll never forget the days(when ) I was in the country.此時,從句 I was in the country已經(jīng)完整,用when增加一個時間狀語.