I'm still struggling to see consistency in the use of case endings. Are they just applied randomly in this course because that reflects real life use or are there some clear rules around their use which I haven't yet spotted in the grammar tips and notes?

Duo is obsessed with the word 'pretty'.

My translation would be "This is a very beautiful language."

Why was 'this language is very pretty' wrong?

I have a problem with this is... versus this.... is