Unfortunately, there aren't any rules for which article to use, so you have to remember each individual case. I think there are some broad patterns, because after studying for a while I found that I had a vague feeling for what felt right, but really you just have to remember each word with its article as a set.
An -en-word is, yes, a word that ends with -en. There are both -en- and -et-words. They exist in both Danish and Swedish, and has (as said) not really any good rules.
It all comes down to the gender of the noun. I have found a blog that might give you an idea with some good examples:
Good luck! :-)
"äpple" uses "ett" because it's an "ett-word"; that's just how the grammar of Swedish is. The same way French (le/la), Spanish (el/la), and German (der/die/das) have gender.
As for English's "a/an" vs "one" usage difference, yes, that's a quirk about English compared to French, Spanish, Swedish, etc. "A/an" was originally "one" and just changed over time.