Translation:The horse, the elephant, and the turtle are eating an apple.
Wow, must be a big apple if it can feed all these animals.
WAIT... Big Apple... THEY ARE EATING NEW YORK!
Until this point, this was a Scandinavian arthouse movie, now it's a CGI-heavy Hollywood blockbuster about giant animals eating New York... What a thrilling ride this Danish course is!
That depends on the company you keep; I use it all the time. It is correct both to use it and to not use it; sometimes you can avoid ambiguity by using it (I invited the ballerinas, Rob Portman, and Barack Obama), and sometimes by leaving it out (I invited my Congressman, Rob Portman and Barack Obama).
It's more common in the UK than the US, from what I understand, but the fact that it's also called the Harvard comma indicates that yes, it does have some traction on the western side of the Pond.
The first image that springs to mind is that they all share one apple. I don't think any Dane would interpret this otherwise.
If they each had an apple, you'd mostly go for the indefinite plural, "...spiser æbler". Or the exact sentence, which sounds a bit convoluted: "Hver af hesten, elefanten og skildpadden spiser et æble." - "Each of the horse, the elephant, and the turtle eats an apple."
it's ridiculous how unforgiving this particular segment is while others would pass multiple et/en/ene/erne mistakes.
I wrote: Hesten, elefanten og skildpadden spiser et æblet.
I don't mind if the whole course asked for perfection, but you cannot have inconsistencies like this where some are forgiving and others are not for the same mistakes.
That is the way Duolingo is programmed: it forgives you typos if the resulting word is not a different valid word in that language. Æble and æblet are different Danish words, so if you write "et æblet", you'll be marked wrong. If you had written "æblen" instead, which is not a Danish word, Duo would have graded it as a typo.