A weakness of Duolingo is the lack of explanations in a lesson. Consequently, we students need to go through the discussions, most of which are a waste of time to find the few wonderfully clear and helpful explanations. If these explanations had been provided at eg the beginning of a lesson there would be much less unnecessary frustration for both the students and the wonderfully patient explainers.
Well this is the way you make a request or give an order in norwegian. If you say "Har en hyggelig kveld!" you are practically saying "Having a nice evening!" which is grammatically and literally incorrect for a Norwegian. You can imagine that imperative forms are not made the same way in english and norwegian language. :)
It's because in English they say "to have" in the infinitive form, and just "have" for the imperative one. The word itself doesn't change. They use the form of the infinitive without "to* to make an imperative. That's why someone get confused, maybe...but in Norwegian the two are simply different words, without any suffix needed to make the difference. Just two different words. Like in latin languages ;)