"Thank you for helping me yesterday."

Translation: Mulțumesc că m-ai ajutat ieri.

[Euratos]

Doesn't "mersi" work as well? Or is it only used as "thanks" instead of "thank you"?

[crbratu]

Yes, it is more informal

[Andy300807]

My answer, "Mulțumesc că tu m-ai ajutat ieri", was marked as incorrect.
What do you guys think? Is it right/wrong and why?

[crbratu]

No Romanian would use the "tu"as it is redundant, just "mulțumesc că m-ai ajutat ieri". But i won't mark your answer as incorrect. You would be understood.

[Andy300807]

Is it worth reporting, then? Since it's correct but not idiomatic.