I’ve learned that both mean something like “I see” or “I understand” but are they interchangeable? Is there any difference?
- そっか is very casual, and it's clearly inappropriate in formal settings. On the other hand, なるほど is safe in business exchanges, but it can sound a little pompous. If a small kid said なるほど, it would sound funny.
- Both なるほど and そっか are used like "Aha, I (finally) got it!", but I feel そっか is mainly used when you have realized a bad thing, e.g., "Oooh, so I was wrong...".
if you add question mark after なるほど, it means “I can’t understand what you’re saying ” or “I can’t agree with you”.
そっか and なるほど is very similar. I think the difference is that そっか is casual response to others but なるほど can be use in both casual and formal situation.
I think なるほど emphasis the point that “ I understand what you are saying”.
otherwise I think そっか is more right response like “okay”.