* 커피를 마셔요? => 커피를 안 마셔요? (short form) / 커피를 마시지 않아요? (long form). * 공부를 해요? => 공부를 안 해요? / 공부를 하지 않아요? * 뭐 해요? (1) = What are you doing? => This can't be negated except in rare cases because it's asking "what"? * 뭐 해요? (2) = Are you doing something? => 뭐 안 해요? / 뭐 하지 않아요? (Aren't you doing [going to do] something?) "뭐 해요?" is most often interpreted as "What are you doing?", but it can also mean "Are you doing something?". This is because 뭐 (무엇) can mean "what" (in a question clause) or "something/anything" (in any kind of clause). In speech, the intonation differentiates the meaning (the latter, "doing something?", has a raised ending), but in writing, it is usually the surrounding context that determines the meaning.