I think your translation is fine; but to elaborate on that:
"wieder" per se only means "again", not "another". I wouldn't put that translation in a dictionary.
Normally, to say that you want "another [glass of beer]", you use "noch ein/eine [thing]" or short "Noch einen/eine/eins (m/f/n)!" (If a child on a playground wants to use the slide "another time", it'd say, "Noch mal!" - das Kind möchte noch einmal rutschen.)
"Ich kaufe nie wieder ein Pferd" can be translated literally with "never again".
The way I see it, if you want to translate "I am never buying / will never buy another horse", there's no other way except with "Ich kaufe nie wieder ein Pferd" as well (provided what you want to say is still that you "won't buy a horse again [in my life]"). There's no German phrase to literally reflect "another horse".
If what you want to say (by "...another horse") is, "Stop suggesting that I should buy a second horse in addition to this one! I won't buy an additional one, never!", it'd be "Ich kaufe niemals noch ein Pferd!", "Niemals werde ich noch ein Pferd kaufen!" or suchlike.
So the question I have, it looks like you partially (if not fully) answered it already.
The way I see it, if you want to translate "I am never buying / will never buy another horse", there's no other way except with "Ich kaufe nie wieder ein Pferd" as well (provided what you want to say is still that you "won't buy a horse again [in my life]").
My first instinct was to translate this into English as "I will never buy another horse." but I opted with "I am never buying another horse." since they used kaufen instead of werden (I've been burned by the owl before). So my question is ... Is it wrong to use werde here? You said the only way to say it is:
"Ich kaufe nie wieder ein Pferd"
No, you're right - you can use the actual future tense ("Ich werde nie wieder ein Pferd kaufen") as well, of course, instead of the present-but-intended-as-future tense ("Ich kaufe nie wieder ein Pferd"). ...at any rate outside Duolingo - I don't know what the owl allows on here. (I know those owl burns, too :) )
Fun fact (sort of): In German, "nie wieder" can't be separated - "nie kaufe ich wieder ein Pferd" doesn't work (well, unless you want to be overly poetic). In my mind, "nie" and "wieder" are glued to each other to form one expression.
("Nie wieder!" is, by the way, a catch phrase that formed after WW II and is still sometimes referred to: "never again" war or fascism in Germany.)
"I'm never again buying a horse" is considered wrong, though it has exactly the same future connotation as the German; both English and German sometimes use the present to stand for the future. But another commenter says "I don't buy a horse ever again" is accepted, and this is just bad English.
The uses of English expressed by native English speakers should be taken into consideration. But my feeling is that the German writers of this set of lessons are absentee landlords, since no suggested changes in the English ever seem to be acted on.
The German sentence is given in the Present Tense. The English translation of "ich kaufe" is either "I buy " or "I am buying". Both are a correct form of the first person singular of the Present Tense of the verb "to buy". However, if "never again" (nie wieder) is introduced into the sentence, no native English speaker would ever say "I never again buy a horse", as Duo insists. "I am never buying a horse again" is a much more natural English translation.
I wrote it wrong the first time and Duo said it should be: I won't ever buy a horse again. When it came up again, I wrote exactly what Duo had suggested, and he said I was wrong and that it should be: I never buy a horse again. Aaargh! How is one supposed to learn the write answer???
Gosh, I wish they allowed non-language-related discussion just now. I'm a horse person, and depending on how bad your car is, a horse might well be more reliable. If you do buy a horse, make sure you go horse-shopping with a knowledgeable horse-person, so as to avoid buying a spoiled old pasture pet. But I better shut up now before I write an entire article. Und ich werde alle Pferde kaufen, die ich will, vielen Dank.
While I agree with you that in most situations it's "very awkward", I'll play Devil's Advocate here and say I think it works in one specific situation. I'm probably going to get the tense wrong, but I think it's acceptable in past continuous?
Basically, imagine a retired cowboy who is detailing his life's story:
"After I broke my back in the Great Rodeo of '65, I never fully recovered. From then on, it doesn't go well for me. I don't buy a horse ever again. I don't get to ride like I did ever again. Heck, I don't get to have fun ever again. Now I'm just drinking my whiskey and biding my time. Oh to be a young cowboy again ..."
That being said, you're correct. It doesn't work in everyday speech and probably shouldn't be acceptable.
I applaud your effort to give an example, but yeah, even this doesn't sound correct. I do like your sentence of "I don't get to ride like I did" but I would stop there and not put the "ever again" on it. But the "I don't buy a horse ever again" still doesn't make sense. I would say in this situation. "I can't buy a horse ever again" or even still "I won't buy a horse ever again". What's funny is that the word "don't" can work if you use is as a command: "Don't buy a horse ever again!"
Actually, as a command, even with 'I' at the front it does work... '...and so there we were, three sheets to the wind and miles from home and I make a mental note... I don't buy a drink ever again, I don't buy a lottery ticket ever again and, more importantly, I don't buy a HORSE ever again!'
True, it gets the message across, but at the same time that argument is a slippery slope. On one hand, language is about communication, but at the same time it's about effective communication.
If you say something grammatically wrong or uncommon, the receiver of the message might spend more time trying to decode it than intended. For a single sentence this isn't a huge deal, but stack sentences up to form paragraphs and so on and it becomes burdensome to the decoder and ineffective.
I say it's a slippery slope because it's subjective as to how many iterations before the sentence becomes unintelligible. Would the following sentence be acceptable to you (assuming it conveys the same message)?
"Never a horse am I buying again."
It's got all the same pieces as yours and I would argue conveys the same meaning, but I wouldn't consider this an acceptable sentence either.
On a side note, keeping the negation with the verb it's negating is pretty important in English (there might be a rule for this but it's been so long I couldn't come up with it if I tried ...) so anything outside of that would throw up red flags because even if it's grammatically correct. It's awkward to hear (sorry if my brethren across the ocean say it like this all the time and I'm just ignorant!).
If you say "I will never buy a horse again" the "never" refers to the act of buying a horse (again). But it seems to me that if you say "I am never buying a horse again" the "never" is implying a continuation of your commitment to not doing so, or to your continuation in the state of not-buying, and therefore the speaker gets away with using the continuous form. It is not very elegant, at the very least, and is probably not even grammatically correct, but it is still a commonly used construction.
Can anyone tell me why the accusative case doesn't apply here?
Why is it ein pferd and not einen pferd?
"Pferd" is neuter gender; the neuter accusative form of the article is just "ein." The masculine accusative form, on the other hand, is "einen": "Ich kaufe nie wieder einen Apfel."