"He is a hard working athlete."
Translation:Han er en hårdt arbejdende atlet.
If you spell it in one word, it is pronounced different. I suppose Duo can't pronounce "hårdtarbejdende" right, and therefore, they have chosen to, unnaturally, spell it in two words. This, however, does not change the fact that it should be accepted to spell it in one word, and that spelling it in one word is the most natural. I have therefore reported it.