Question

I still remember the time -we spent together at xishan mountain last year.

A when B what C who D which 为啥选D

Answer 1

考察的是定语从句，BC可以直接排除，纠结在A和D。这句话的先行词是the time，因为后面跟的是spent这一动词，所以你不能把the time 看成是时间，而应该看成是某物。spend sth on sth
或者spend sth(in) doing sth。所以用which代替先行词。

换一种情况，I still remember the time -we “are” together at xishan mountain last year.这时候就要用when.

解释得不够专业，不知道你能不能明白~~希望对你有帮助！