I don't understand why sometimes an e is added to the end of a word. Is there a way to know when the e needs to be applied as a suffix to the end of a word?
-e is added to most adjectives when they are plural, and also in the definite when they are attributive (come before the noun)
If you're not aware, there are notes for each skill on the website (but not in the app). Click on the skill, then on the lightbulb.
I read "Replay" by Grimwood recently. I recommend it to you, reading this. It's a time-travel book, recommended by Seth Godin as "the best travel-book ever written", I was not disappointed. Pretty original and touching.
Okay so the adjective flott means great ,but in the sentence det er flott there's nothing added for the neutral gender
Yes, that's right. "Flott" changes to "flotte" when it's plural (e.g. tre flotte bøker) or definitive (e.g. den flotte boken), but since it already ends in "t", it doesn't change when it's in the neuter form (e.g. det er flott). Was that the change you meant?