Sometimes I think how grateful I would be today if I had learned more back then about what really matters.
back then=at that time。back是形容词，then是名词吗？把back then放到句尾会有歧义吗？
假如必须要剖析的话，那样，back 和then 都是副词。
其中，then 是“中心词”，then = at that time 之意。
back 修饰then，起强调用途，指“时间或地方”的“追溯，上溯；回原处”。