【解答】
1. I'll never forget the time(which/that)I spent in Beijing. 这句话中,the time是先行词,which作the time的关系代词,之所以不能用when是因为在从句中,I spent ____ in beijing中spent的宾语是缺失的,所以这里需要which或that充当那个宾语的成分.而when,where,why引导的定语从句中,三个关系副词后的句子成分是完整的,不缺失任何成分,所以用这第三个词是没问题的.
2. I still remember the time when I joined the League.中,when后的句子成分是完整的,参考1中所讲的,就顺理成章了.
希望可以帮到你.