This matter is brought up in a few places. Here's a link: https://forum.duolingo.com/comment/32896328 There are other places, too, so you could do a search to see other responses to this good question. Here's another place: https://forum.duolingo.com/comment/32830304 Since DL Arabic is in Beta form, there are some glitches. If you get an introductory Arabic grammar, look under nunation or tanwīn. Apparently it will become clearer in time as we progress. Peace.

ك this one