Duolingo is usually very good, but occasionally seems wrong. My German course produced an apparent mistake today:
it asked me to translate: that is the second example
I wrote: das ist das zweite Beispiel
It marked that as wrong and corrected to: das ist das zweiter Beispiel
I have checked with a German speaker, who tells me that mine was the correct version.
When you complete a sentence/exercise, and Duo marks you wrong but you are right, if you look in the bottom left corner where Duo shows you the correct answer, you'll see a button where you can report an error. Click that button and you'll see a list with check boxes. One of them says "My answer is correct." Click that and submit the problem report. The report goes to the course contributors, and they read ALL of the reports. They do not, however, read all the discussion posts in the forums here nor in the sentence discussions.
I have reported the error, I loose health and the game continues with the same question. I am stuck on Phrases 2 the last question.