"There is a princess with a long dress outside the door."
Translation:Det står en prinsessa med lång klänning utanför dörren.
Either way works. We sometimes skip the article for clothing, e.g. en gubbe med hatt 'an old man in a hat'. I think it's because clothing is something we expect people to have. We don't usually focus so much on the individual item of clothing, that it is a dress, it's more about either it's presence, or about some characteristic of it. If we look at the reverse situation, utan, e.g en hund utan svans 'a dog without [a] tail', it's much more natural to skip the article. en hund utan en svans might not be totally wrong, but it sounds off for most contexts. And in this case it's clearly about the absence of the thing, and the thing in itself doesn't matter.
Also for body parts. en gumma med stor näsa 'an old lady with [a] big nose' is more common than the version with en näsa. It is to be expected that the old woman has a nose, so we don't need to point out that she's got a nose.
We always use the article in cases like these though IF the item isn't where it's expected to be. en prinsessa med en lång klänning i en påse 'a princess with a long dress in a bag' or en flicka med ett stort huvud på ett fat 'a girl with a big head on a plate'.
In principle yes, but when speaking about one single person or several people who are only temporarily in a place, we normally choose a position verb. In this sentence for instance, it would be a bit weird to inform someone that det finns en prinsessa … – it makes it sound as if she's always there or something.
But if you say Det finns många finnar i Sverige for instance, 'There are many Finns in Sweden', that is perfectly normal. Or like, Det finns en tjej i min klass som … 'There is a girl in my class who …' – her presence there is more permanent.
So does this make "det finns" incorrect or just really abstract in this case? It's marked as incorrect.
It's odd about a person who's only there temporarily, so it doesn't fit here. Not necessarily super wrong, but too wrong to be an accepted answer.
Why not finns en prinsessa - because there is not extra info about whether its an abstract "there is a princess" or that the princess is right there standing?