"¿Para qué quieres más papel?"
Translation:What do you want more paper for?
Technically "What do you want more paper for?" is grammatically incorrect. Strictly speaking ending a sentence with a preposition is wrong. However, just about everyone talks like that, so it is OK for Duo to use it. The correct, although seldom used way of phrasing this would be "For what do you want more paper?" This sounds pretty awkward; a more natural translation would be "Why do you need more paper?" Duo doesn't accept either of the last two translations.
Like you, I thought this to be the rule. In actuality, this is an extension of a rule for Latin and erroneously extended to English. For reference: https://www.merriam-webster.com/words-at-play/prepositions-ending-a-sentence-with
Cool, thanks. I’ve pretty much forgotten my Latin. I probably find this picking apart of languages far more interesting than I should!
"Why do you need more paper?" = ¿Por qué necesita más papel?
There is a difference between why do you need something (por qué) and what do you need it for (para qué). It's a subtle but important difference. However, it can help you to start to learn the difference between para and por. Most of us need all the help we can get with that.
Para qué is not as common, but in this case, it means what exactly are you going to do with this paper? What is the purpose for the paper?
The answer could be "I need the paper in order to do my homework" or to cover my walls with paper, or whatever it is that I am going to do with it. That's the purpose (con cual fin) .
Por qué is asking what is the reason that you need the paper . Why do you need the paper? What caused your sudden need for paper?
The answer could be "I need the paper because my teacher assigned us homework that is due tomorrow and I've run out of paper." That's the cause (con cual causa).
While I do not know all of the subtle distinctions between para qué and por qué in Spanish I see "Why do you want more paper?" and "What do you want more paper for?" as equivalent in English in terms of meaning but the first is far more natural (and wrong according to DL) My sentence used quieres not necesitas but to me the same holds true for need. I think this is a case of DL trying to force a distinction that exists in Spanish between para qué and por qué into a English where due to the flexibility of the language "Why?" and "What for? are used equivalently at least IMHO. If there is a distinction in English then it more subtle that the distinction between "shall" and "will". :-)
No! Nobody talks like that. You might hear “what for?” as a synonym for “why?”, but only standing solo. Not in a sentence like this.
Ruth-Ellen4, I'd have to disagree; I've heard plenty of native English speakers use this "What....for?" construction. "What do you want to go there for?" "What are we doing this for?" It does sound less educated ("correct") and more abrupt or even rude, but it is well-used.