I will never forget the days when we were together and which made me really happy.

参考译文：我永远不会忘记我们聚在一起并使我很快乐的那些日子。

解答：以上是语法结构分析如下

【1】主句：I will never forget the days.【will forget为谓语动词】

【2】定语从句(1):when we were together. 【were是谓语动词】

备注：关系副词“when”引导定语从句。修饰名词“days”关系副词when在定语从句作时间状语。when=on which

【3】定语从句(2):which made me really happy.【made是谓语动词】

备注(1)：关系代词“which”引导定语从句。修饰名词“days”,关系代词which在定语从句作主语成分。which代替先行词the days。

备注(2)：由并列连词“and”连接的两个并列定语从句同时修饰前面的先行词“the days”。

【特别备注】以上英语句子有3个谓语动词，2个从属连词。and为并列连词连接2个并列定语从句。