Sometimes they just haven't added a variety of formalities into the accepted answers! Use the flag and they'll review it. :)

well, I know what you mean. I am only starting out on duolingo but you must understand that some of this is not always going to be formal, I have learned differently and I guess some of the words that I learned years ago were wrong. My point is that Korean is a very difficult language to learn/understand but you should just empty your mind and start over. A blank slate might help with gathering more info about this language. I have been practicing Korean for over 3 years and I still don't have a clue what some of these words say :)