error in grading/translation
When Duolingo (app) makes an error in grading/translation, how to I report it?
The iPhone app doesn't currently have a way to report these types of errors though the app. Your best bet is to probably redo the lesson on the web, make the same mistake, and report it there unfortunately.
Yes, very obvious. When you get an answer wrong, at the bottom left of the screen (under the correct translation), you will see two options: Report a Problem and Sentence Discussion. When you click Report a Problem, one of the options is "My answer should be selected". Just select that option and click submit. That's it!