Hello, we have a misunderstanding going on here.
I only meant to point out that when "ei mus" is used, it should be "ei mus, musa, mus, musene". You can search it up on Wikipedia (en mus bøyning). When it's "en mus" it should be "musen", that is correct.
I hope I managed to make myself clear.
As I answered hereabove, this would be "Musen spiser ost". "Osten" has the definite article (-en), so it is specifically "the cheese". "The mouse is eating (or "eats") the cheese" is therefore the only correct translation. All the Duolingo Norwegian course is very strict with regard to this. One of the main aims of this course is to teach us to really pay attention to the difference between definite forms (with articles, like -en) and indefinite forms (without articles) in Norwegian.