"His radio is especially slow."
Translation:그의 라디오가 특히 느려요.
Why not "그의 라디오는 특히 느려요." Is there something in the grammar that disallows 는 or is it just duolingo messing up again?
Either could be right, depending on the situation. DL needs to provide context in order to really teach how the topic and subject particles are used. 그의 랃이오가 특히 느립니다. should also be accepted 1/9/18. The degree of politenes should also be hinted at if they are only going to accept what the developer conceived as the "right" answer.