Some languages have one word for rats and mice. English has different words for rats and mice. English distinguishes apes and monkeys. The French call them both "signes", but they have one word for owls with ear-like tufts on their heads and a different word for owls without those tufts. The English call them both "owls."
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.