"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'.
I also answered "med en lång klänning", based on the wording of the English statement. If I hear "a princess with a long dress", I don't imagine the princess wearing the long dress, but carrying it. I would expect to say/hear "a princess wearing a long dress" for that. With that in mind, I translated it to mean that somehow the princess was carrying it, but without specifying how she was carrying it. Would saying "med en lång klänning" be a suitable translation if was known that the princess was carrying the long dress, but not the manner in which it is being carried? Or should one still skip the "en"?
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.
I know it is not not normal in this computer age to actually read information as everything is reactive, in fact over reactive. I find that in many cases you have provided the answer a number of comments previously but the same question is repeated a number of times. May I therefore encourage my fellow students to search the detail before adding to the comments. Love and kisses to all ( as appropriate) Alan.
You explained quite well why "står" works, but the original question is still unanswered: why is "är" not acceptable?
EDIT This was explained by Arnauti in the first comment. A user asked if it was ok to use the verb "finns". The answer is no because it implies an action going permanently or for a long period of time (read up the first comment for further details).
Saying "Det finns/är en prinsessa med lång klänning utanför dörren." means that the princess is ALWAYS staying outside the door. Well, of course she could be an extremely creepy princess but you get my point.
Are you picturing something like this? https://en.wikipedia.org/wiki/Litter_(vehicle)#/media/File:A_History_of_Madeira,_1821,_P_107.jpg
To reprise what wiser people have written higher in this thread: Det finns is more for something permanent or generally true. (As I am only just getting this I can't give examples and be sure I am right. It is just another interesting case of one language, and it's use, not matching to another.)