Most Korean learners are confused to tell the difference, but check the answers following the questions. "WHO" ate my pizza? > I did.(not others but "I") = 내가 먹었어(O) / 나는 먹었어(X) "WHAT" did you EAT? > I ate the pizza. = 나는 피자를 먹었어(O) / 내가 피자를 먹었어(X) (I ate something which is the pizza) \^o^/