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是形容詞，then是名詞嗎？把back then放到句尾會有歧義嗎？
假如必須要剖析的話，那樣，back 和then 都是副詞。
其中，then 是“中心詞”，then = at that time 之意。
back 修飾then，起強調用途，指“時間或地方”的“追溯，上溯；回原處”。