"I am sorry."
Gomen nasai = "I feel bad about it", average level of politeness Gomen, gomenne (masculine), gomenna (feminine) = "sorry dude", just for friends and often jokingly Sumimasen = "it is my responsibility", high level of politeness, can also be used as "excuse me" to formally attract attention.
gomenasai and sumimasen are the same when being sorry, and both are formal, believe me you can say both when you are sorry. the informal ones are suman, and gomen, gomenna gomenne, belive me there are also the highest politesness they used in japan, especially when saying sorry to customers or very high people. but for now this is good, if you want to know what those are just comment down.
In my experience DuoLingo doesn't teach you anything, not in Japanese that is.
I think sources like Human Japanese and TextFugu do a better job at making sure you truly understand the language, and didn't just start memorizing random things based on practice. Don't get me wrong though, I really do like DuoLingo for rehearsing what I've learned in the past year!