It's not so much after tragen but before Hüte.
kein is more or less the negative version of ein -- an article that goes before a noun.
The plural version keine is similar to the English "any", which also acts as a kind of negative indefinite plural article, as in "Cows do not wear any hats" -- the "any" goes with the "hats". But while English needs a "not" together with "any", keine already contains the negative meaning inside it.
Also, tragen as the conjugated verb has to be the second thing in the sentence, so it comes right after Kühe.
The rule is to use "kein" instead of nicht, when you negate a sentence with an indefinite (i.e. with an indefinite article or no article at all) accusative object. So it is:
"Der Hund bekommt einen Knochen" --> "Der Hund bekommt keinen Knochen"
"Der Hund bekommt den Knochen" --> "Der Hund bekommt den Knochen nicht"
"Kühe tragen Hüte" --> "Kühe tragen keine Hüte"
"Kühe tragen diese Hüte" --> "Kühe gtragen diese Hüte nicht"
Not if you want to negate the sentence as a whole. If the sentence has an indefinite accusative object (i.e. one with either an indefinite article or no article at all) a sentence is negated using "kein", not "nicht".
Here the positive sentence would be "Kühe tragen Hüte", "Hüte" is an indefinite accusative object, hence "Kühe tragen keine Hüte".
There are two exceptions to this rule:
1.) If you move some part of the sentence to the very beginning, in order to emphasize it, then "nicht" comes in again, e.g.
"Hüte tragen Kühe nicht" (note the stress on "Hüte")
2.) If you want to particularly negate one part of the sentence in order to contrast it with other options. In this case you put the "nicht" directly in front of the respective part of the sentence, but you have to give some continuation then, e.g.
"Kühe tragen nicht Hüte, sondern Kappen" = "Cows don't wear hats, but caps".
Is there a difference between "don't wear" and "are not wearing" in German?
In the standard language, no.
However, note that this is a general sentence -- it talks about cows in general. So in this case, only the present simple is appropriate in an English translation: not because of how tragen can or can't be translated, but because of what verbal tense is appropriate in English when talking about general truths.
If we had been talking about a specific group of cows, then both "the cows are wearing hats" (= at the moment) or "the cows wear hats" (regularly) would be appropriate as a translation of die Kühe tragen Hüte.
But für Kühe tragen (keine) Hüte, only really "Cows (don't) wear hats" makes sense, because of how English grammar works.
"cows do not wear hats" should be accepted.
However, there was a report from about the same time that you posted this comment asking for "cow do not wear hats" to be accepted. Was that what you had typed?
Singular "cow" is not a suitable translation for plural Kühe.
Screenshots are helpful in diagnosing this sort of thing :)
The next time you run into a translation that was marked wrong and you're not sure why (error in the program, missing alternative in the system, typo on your part) -- if you can, take a screenshot, upload it somewhere, and past a link into your comment.
Dear Duolingo Team, you urgently need an AI in your app. So, I wrote Hütte (houses) instead of Hüte (hats). Yes, it's an error, but it's also a typo as it is in Kuh - Kug. Your app thinks all the two are words, and your app notices typos only in 1 word, on that one who isn't another real word, getting? Thanks, may the AI be with you!
Yes, that’s a limitation of Duolingo: if the result of a minor typo (one letter or so) results in a non-word, then you get a typo warning, but if it results in a real world, Duo can’t tell the difference between a typo and a mistake (i.e. you think that the German word for “hats” is Hütte and typed that word intentionally).
An AI that actually understands German would indeed be awesome.
How much are you willing to contribute to the programming costs?
Why does it say "do not" as wrong, but "don't" as correct?
"do not" and "don't" should be accepted interchangeably.
Perhaps you made a mistake your first time?
A screenshot would be helpful to let others see what happened -- upload it to a website somewhere, please, and tell us the URL to the picture.
"Cows are wearing no hats" (and also "Cows aren't wearing hats") isn't a correct English sentence, because the idea is that cows never wear hats, in general. So you'd need to use simple present: "Cows wear no hats", or (more elegantly, I think) "Cows don't wear hats". German: "Kühe tragen keine Hüte."
If it's "the cows", you can use present continuous: "The cows aren't wearing hats", "The cows are wearing no hats"; German: "Die Kühe tragen keine Hüte". This would mean that only these cows are not wearing hats now.
But the sentence isn't "cows never wear hats", it's "cows don't wear hats". (No neimals). However, there has to be a way to say "cows are not wearing hats" in German and if you wanted to say that, this would be the way, wouldn't it? If they are going to ask you to translate silly sentences without context, then all silly viable translations should be accepted.
Of course there is no "niemals/never" in the sentence. But nevertheless it is a general statement that applies all time. What should "Cows are not wearing hats" even mean? I can only imagine very funny settings, where this could work. Maybe something like: "There is a big party on the farm. All the horses are dresses as red indians and are wearing feathers in their manes. And all the pigs are wearing basketball caps. But cows are not wearing hats ..." Doesn't work there either, should be "The cows are wearing hats".
Could you give a better example?
But even if I could accept something like "Cows are not wearing hats today" the German sentence would still be "Kühe tragen heute keine Hüte".
F: "Beth, Don't you think the cows ought to be wearing hats for our shindig? After all it is a party."
B: "Fred, the cows are NOT wearing hats. That would be rediculus."
F:"But we're dressing the horses like Indians."
B:"Fred! I said, the cows are not wearing hats. We already decided they're wearing bows."