"Write on the paper."
Translation:E kākau ma ka pepa.
Late reply (school starts in two days!), but I would stick with "ma ka pepa." In fact, I would prefer "E kākau ma luna o ka pepa." Something to tell a student who is writing on the desk. With "i," it could also be an object marker in case the student is supposed to be writing a paper for class but he's playing around. And besides, "kākau i ka pepa" just doesn't SOUND like "write ON the paper." But you probably already know all this by now since your comment was written 8 months ago. lol