Here is the single best version of the truth, and never ask again !
Nouns are always one of both cases. A good dictionary is the only one telling you the truth.
We have a fex dictionaries listed here => viewtopic.php?p=2821#p2821
So in the end you'll have to know the article by heart for all the nouns in your vocabulary.
It may thus help to organize your flashcards this way
- en/ett
- noun singular, noun plural
- definite singular + plural
examples:
ett äpple, äpplen, äpplet, äpplena
= an apple, apples, the apple, the applesen bil, bilar, bilen, bilarna
=> a car, cars, the car, the cars
What to do if you encounter a new word? Well you can guess about en/ett,
and there is a good chance that you get it right, as by far a majority of nouns are en- nouns.
The other ones are ett- nouns, right?
So, If you were to guess, you may need to follow some guidelines described here.
The idea being that often the ending of a noun is related to being an en- or ett- noun
You find list of endings, as well as most common exceptions, in this archive
https://archive.ph/ja5yc
Basler Biker
/ fluent 
/ learning 
/ fan of 
(bs/bl)