This question was a year ago, but if anyone is having the same problem (as I was until I stumbled on them) then: If you can open a browser, go to the Duolingo home page, sign in, then select home from the dropdown menu. You will see your current language tree. Click on the circle for the subject (here it's "Adjectives".) Not every subject will have Tips and Notes.
I wonder why in this sentece the 's' is not pronounced as a retroflex, although in other sentences it does. I am aware that the sound change doesn't happen across words in lots of dialects, but in the Oslo one (which is afaik the one Duolingo uses) it does. Is there a reason for this?
Most likely just because of the way it was recorded and then made into sentences. I think that's the reason. And because as you said there's lots of dialects of equal importance it is not a mistake by any means. I'm not native not even fluent so take what i say with a pinch of salt but I think that's all there's to it.
I am far from the pro, so maybe just get someone else to confirm this but I think it works the following way:
a child is hungry - et barn er sultent
the child is hungry - barnet er sultent
children are hungry - barn er sultne
the children are hungry - barna er sultne
the mouse is hungry - musENE er sultne
and then you go into the, so called, double deffinites:
the hungry child - Det sultne barnet
the hungry mouse - Det sultne muset
the hungry children - De sultne barna
the hungry mice - De sultne musENE
a child is a bit tricky to conjugate, because in plural deffinite it doesn't get the typical "-ene" ending.
hope, this helps :)