A basic guide to pronunciation can be found in the tips and notes for the first lesson, see https://www.duolingo.com/skill/nb/Basics/tips-and-notes
Regarding k and sk, it seems they get softened when followed by an i, y or j but stay hard when followed by any other letter. So when saying for example 'skitne sko' ('dirty shoes'), the 'sk' in 'skitne' is softened to a 'sh'-sound because it is followed by an 'i', while the 'sk' in 'sko' stays a hard 's-k'-sound because it is followed by an 'o' and not by 'i', 'y' or 'j'.
i believe this is just official spelling based on pronunciation, the "t" in "skitne" is more subtle than the "t" in "skitten" so it only gets one "t", or at least that's the way i think of it. As far as i can tell Norwegian official spellings are a lot easier than English official spellings and you seem to have English down pretty well, so you shouldn't have too much trouble getting "skittne" out of your head.