Both your sentences "I eat the apple" and "I eat apple" are OK in danish; they have slightly different meaning. If you say "han spiser æble" without the 't' at the end, you are stating that he is eating some unspecified amount of apple, or that he is not eating other stuff (like pear). When you say "han spiser æbleT", it is the specific apple that he is eating.
It's probably good to point that out, but that's not was he was asking. I honestly cannot hear the difference either and I'm not new to studying languages (not Danish obviously). I think we just want clarification on if there is meant to be a consonant of some sort at the end and the rules in Danish for pronouncing consonants.
Yes, the -t at the end indicates that it's THE apple. (Because apple is a neuter word. If it had been common gender, the ending for "the" would have been -n.)
Danish also has a separate word for THE, but it is not used in this sentence. One place you might see it is if there is an adjective before the apple, e.g. det store æble "the big apple", where "the" is expressed as a separate word det rather than as an ending -t on the noun.
Some Scandinavian languages use both, e.g. Swedish det stora äpplet with both det and -t.
Maybe you mean the clubs? But it'd be great if duolingo had a feature where you could just talk to the people you want, just hitting them up, like dms on instagram (that I don't even have but I suppose it'd be similar?) You could speak to the people you ''friended'' on here, and the natives. And I don't mean changing the rest of the app, just adding this. It'd be so cool