"Elephants eat apples."
Translation:Olifanten eten appels.
Okay, so what is the difference between appels and appelen? It seems like they would both be plural, but should one be used differently than the other?
Oftentimes it's a matter of style and common practice. The way I feel is that -en oftentimes sounds more formal and masculine, whereas -s sounds more informal and feminine. For example, I would interpret studenten as either a group of male or mixed-gender students, whereas I would interpret studentes as a group of female students. Sometimes an s-sound is more easily heard than an n-sound as well (e.g. ziekten vs. ziektes) or just sounds less awkward. In other cases there is no other way than to use -s, oftentimes with loanwords (e.g. garages).
There are plenty of rules and therefore also exceptions, however. For now, I would say it is safe to stick with -en as much as possible. If Duolingo teaches you to use -s, I suggest you pick up a Dutch dictionary and see if it's the only possible ending. This way you'll slowly build up a feel for it.
Well, you're quite right about the studentes-studenten thing, but please, never say 'appelen', always use 'appels'. If you'd say 'appelen' you'll sound like the king's grandmother, since it is that old fashion and posh.