Oh, completely missed this part at first.
This has to do with the genders of the nouns. --en is male, --et is neutral, and --a is female! (So the same difference between “le” and “la” in French, for example.)
This sounds like a very complicated language now, but it’s not really. For example, the verb “to be” is just “is” for everyone, so verbs at least are very simple.