"Norwegian is easy to learn."
Translation:Norsk er lett å lære.
"Lett" and "enkelt" are usually interchangeable when meaning "easy", though "lett" has several additional definitions. Technically "lett" is the closest to "easy" in meaning, while "enkelt" is closer to "uncomplicated", but those two definitions usually coincide.
"Greit" is a little more tricky. It's also used along the veins of "easy", "uncomplicated", as well as "clear" and "straightforward", but is used differently in different contexts. You may say that "Eksamenen var grei.", meaning that the exam gave you no trouble. Yet that doesn't necessarily imply that the exam was super easy. Or that "Han var grei å forstå.", meaning that he was easy to understand, likely because he spoke and formulated himself in a clear and understandable way.
"Greit" is in many other contexts used to mean "okay", "fine", or "passable". The bottom line is that it's a pretty hard adjective to get a grip on, so you're safer with the other two unless you've already seen it used in the context you're about to use it in.
so, i can use this anytime i don't want to repeat the same sentence? For example, someone asks me a question: ''Er det enkelt å lære engelsk?''. I can answer : ''Ja det er det.'' Correct if i'm wrong and sorry if i'm being too annoying...Just trying to learn it correctly ahah.