Some words change when they become plural. e.g Man = Mann. Men = Menn. Duck = And. Ducks = Ender. It's just another fun little curveball Norwegian throws at us.
I don't know how to explain the second part. But it's like.
Woman = Kvinne Women = Kvinner The Woman = Kvinnen The women = Kvinnene
I don't know enough of the language to say why it's that way, but it just makes sense you know? Kvinnene makes sense to me, because its a plural of THE women. I'm sure someone else will piggy back to explain it better.