Icannotforgetthedayswespenttogether.(that,which,/)Icannotforgetthedaysweweretogether.(when)为什么第一句句可填that,which,下句却是填when,两句有什么区别吗

Question

I cannot forget the days _____ we spent together.(that,which,/ )
I cannot forget the days _____ we were together.
(when)
为什么第一句句可填that,which,下句却是填when,两句有什么区别吗

Answer 1

两句话的意思是一样的,主要是要看定语从句
1we spent together spend(spent)是及物动词后面要接宾语,所以that/which充当从句中的宾语
2 we were together 是一个完整的句子是系表结构的,所以用when在从句中不充当任何成分
that/which在定语从句中充当成分==从句没有主语or从句中缺宾语
where/when一般不充当成分