Translation:De paarden eten een paar appels.
Why is "De paarden eten een weinige appels." wrong?
It should be weinig since it is not used with "het" or "de", but weinig is also not accepted. If the context means "not much", and it can be with a sentence like this, it should be accepted in my opinion.
It didn't mark "appelen" as incorrect. Are both appelen and appels perfectly interchangable or is there some small difference?
Both are plurals of appel, appels is more common than appelen though.
so based on the convention, am I correct in surmising that "appel" is a "de" noun?
Indeed! Edit: 'de' is the article for all plurals, whether it is a 'de' or 'het' noun.
That does explain a few things …