"Lunch is a meal."
Translation:Lunsj er et måltid.
Why 'en' is wrong? How do I understand when I have to use EN or ET ? Thanks
because måltid is a neuter noun and in Norwegian the article has to agree with the grammatical gender of the noun it is paired with. Take a look at the info in the first section of the first lesson: https://www.duolingo.com/skill/nb/Basics
If you take the time to learn the grammatical gender of nouns at the same time as you learn the nouns themselves, your future self will thank you.