no...whether it is "et" or "en" depends on the grammatical gender of the noun in Norwegian
So because "eple" is a neuter noun, it has to be "et eple"
It's a good idea to try to learn the grammatical gender of Norwegian nouns at the same time as you learn the meaning of the noun itself.
yes...nouns in Norwegian have grammatical gender. For most nouns it's not possible to guess the gender just by looking at it. That's why it's best to learn the gender of the noun at the same time you learn the noun itself. There is more information at https://www.duolingo.com/skill/nb/Basics