There is a slight change in meaning, or rather, the questions start from different assumptions:
‘do you have an idea or nor?’ (‘hast du eine idee oder nicht?’) is a general question, where ‘or not?’ is just there to reinforce the interrogative meaning. The speaker doesn't know what the answer is going to be.
‘but you have an idea, or not?’ (‘aber du hast eine Ahnung, oder nicht?’) or, better phrased, ‘you do have an idea, don't you?’ is a statement followed by a question, signalling that the speaker was reasonably sure that, indeed, you have an idea, but later developed some doubts. The expected answer is still positive, the question is just asked to look for reaffirmation of what the speaker already believes true.
The main point of the lesson is the difference between:
"Hast du ..., oder nicht?" = "Do you have ..., or not?"
(Normal or blunt question. Answer could be Yes or No.)
"Aber du hast ..., oder nicht?" = "But you have ..., don't you?"
(Polite question. Expected answer is Yes.)
I am not an expert in any capacity, I am giving my input as a user and (non-native) English speaker just as you or anyone else is doing. Slight differences have always been of import in Duolingo's teaching method, because teaching by example means the translations have to be as close as possible to the original to allow users to pick up on the fine points that would otherwise be covered in more extensive explanations. Moreover, I was really using ‘slight’ as a courtesy to those who seem to think there is no difference at all, conceding that it might indeed appear negligible to some, but that to me a question and a statement are very different things. Given how the grammars of both languages allow to make this distinction unequivocally, I don't understand why we should muddy the waters by allowing imprecise translations of this kind. ‘You do have an idea, don't you?’ and ‘Do you have an idea or not?’ are, in my opinion, two separate sentences, and if a user hopes to learn German they should know this difference is made in the target language as well.
This whole paragraph, as the comment above, is of course only my opinion and I haven't prefaced every sentence with this disclaimer just for the sake of conciseness.
What you say about the differing English translations is correct. But the issue is which one does the German example mean. Duo says and you seem to agree that the literal interpretation is incorrect.
Even though the German words literally put an obligation on the listener to respond with whether he does or does not have an idea, the actual take away is that the listener does have an idea. He can simply nod or even make any basic sound that does not convey negative and the speaker will be reassured.
In the former case, do you have an idea or not the listener responds with....Damn right I have an idea and I resent your implication that I might not....
In the latter case of But you have an idea don't you the listener can respond with ......mmmmmm..... and the speaker is satisfied. .
I don't understand what is giving you the idea that either me or Duo are against a literal translation: both my and Duo's translations take a statement for a statement and a question for a question. The original commenter instead chose to translate the first statement with a question, which is what I objected too.
So, in short: according to both me and Duo the correct translation is “but you have an idea, don't you?”, which is a very literal translation.
Translating oder nicht as don't you doesn't seem very literal to me.
As you point out in your comment, there is a significant difference between the two sentences.
In English, Do you have an idea or not is a challenge to the listener, almost rude. In fact, in most circumstances it would be taken as rude.
Conversely, you have an idea, don't you is much softer starting out by saying the listener does have an idea, and then asking for confirmation. ////////// Edit:
I have been doing considerable reading in German since this discussion started and I have come across several such instances of or not/oder nicht where the phrase is self directed toward the speaker not the listener. It qualifies the positive nature of the sentence by the speaker.
We will meet at nine at the restaurant..oder nicht/right? But you do have an idea, oder nicht/right? The speaker has just made a statement and then qualifies it by saying oder nicht.
In the examples I have read ...But you have your passport oder nicht?...doesn't mean ...Either you have your passport or you don't... It means ....But you have your passport, or maybe you don't.
Rhetorical questions are questions nonetheless, so they still need a question mark and my proposed translation is exactly the same Duo suggests. The intention is having the question mark refer to the last part of the sentence, rather than to both parts (as evidenced by the grammar used in the first clause). Still, I can see where the confusion is coming from. I hope I helped clearing that up a little bit.
We're focusing on different parts of the sentence: the best way to translate ‘oder nicht?’ depends on the way you translate ‘du hast eine Ahnung’. If you translate the latter ‘grammar-for-grammar’, i.e. as a statement: ‘you have an idea’, translating the second part as ‘or not?’ is forced, while ‘don't you?’ would be the idiomatic expression used to ask a negative question after a statement. This is why I refer to this (perhaps choosing my words poorly) as ‘literal translation’. An entirely literal translation (‘but you have an idea, or not?’) sounds off in English and the closest thing that I can come up with is what I'm proposing.
Translating the first clause as a question (‘but do you have an idea’), on the other hand, changes the grammar and possibly the meaning of the sentence, framing the statement as a question. This does allow a more literal translation of ‘oder nicht?’, but it completely changes the sentence's structure and I consequently don't think it should be accepted.
I see your point. The thing that threw me off is the Duo example has a question mark so I took it as a straight question.
As a question it seems to express exasperation in English. But, do you have an idea or not?
But as you point out as a sort of rhetorical question/statement thingy it is more along the lines of seeking confirmation. Thus, But you have an idea, don't you is more appropriate.
I think that's more a question about English. I don't see a difference in those translations, but I'm not a native English speaker.
Anyway, we sometimes build questions in German by simply stating something and then adding an ", oder?" or ", oder nicht?". (e.g. "Wenn Pferde rückwärts laufen, dann nennt man sie Drefp, oder?" - When horses go backwards, then you call them esroh, don't you?) English uses this slightly more complicated structure for this... if I recall correctly.
Maybe you've shifted the feeling a little by adding an unnecessary "do," but I would report this, if you didn't and have another opportunity. Since German doesn't have a "do" helper, it probably just didn't occur to the course contributors that it could be said with one.
EDIT (2 months later): Your suggestion has now been added to the list of correct answers.
In British English the tag question should mirror the question in the main clause. You have: haven't you? You did: didn't you? There is some American influence on some speakers, but it is not widespread. The inventiveness of American English is often a thing of wonder, but we are not yet there with You have: don't you? Still less with confusing constructions like "You don't got it yet, do you?" My old English teacher would have heard squeaky chalk scraping across blackboards at that one!
Please tell me if I am wrong. I see a real difference between "don't you" and "or not" since for me, don't you means "or you don't have an idea" (in german we would use "nicht" ) whereas "or not" means more "no idea" (in german we would use "keine Anhung" ). Thinking that way, translating "oder nicht" in this context as "don't you" is more narural to me.
PS: Edit: finally I disagree with what I wrote..sorry.. please note that I am not a native english speaker.
I applaud your trying alternatives. I do that too, when I'm translating English to German. Sometimes Duo's lack of flexibility is a mistake, and sometimes it reflects the fact that the alternative is incorrect or not as good. "But you have an idea, no?" falls into the category of "not as good."
Okay, first of all, I wrote "But you have some idea, right?" and it was marked wrong. I know that "eine" = "an," but it sounds just as good in American English (don't know about British or other versions) to say "some." What do other native speakers think? Also, I've read some of the other comments and I think "or not" should also be accepted.
The best translation, imo, is Duo's: "But you have an idea, don't you?" The question then is how far to depart from the optimal translation. "Some idea" is less general than "an idea." "Some idea" applies better to having ideas about a solution to a problem. "An idea" applies to solutions and equally well to creative thoughts unrelated to problem solving. "Don't you?" expresses a positive expectation with no pressure. "Right?" expresses a bit of pressure. "Or not?" is more challenging—which apparently "oder nicht" in German is not. In general, I don't have a strong opinion about how loose Duo is about these points. For myself, however, I prefer being steered to the best translation over being told I'm right, if there's a reason why another answer is actually better.
"Either you have an idea or not" seems the best answer to me. I understand that "aber" is "but", and not "either", but as an expression, "but you have an idea, don't you" is something I can't imagine ever saying in English, even given the potential contexts listed below.
In just about any situation it seems to me that in English someone would say "Either you have an idea or not", or "Either you have an idea or you don't". Sure, I can translate this literally from the German to "But you have an idea, don't you", but it seems very unnatural.
That is a completely different idea though. “Either you have an idea or not” is not asking the person whether they have an idea, it's just stating the fact that you can either have it or not, but no middle ground, as if the person had somehow said or implied that they have half an idea, which is not at all what the German sentence is implying.
The German sentence sounds to me as if the speaker were talking to a listener that is reluctant to express what they think about the current situation, even hesitating to say they have any thoughts about it at all. I can imagine the rest of his line being something like “yeah, you say you don't really know, but you do have an idea, don't you? Come on, speak your mind!”.
Is the proposed translation the best or even an acceptable translation at all? Probably not. But I personally cannot think of a more fitting translation anyway.