Hi Julia, As I see it, the problem is with the particle. Here you use the object particle 을/를 but you need the subject particle 이/가 or the topic particle 은/는 because 'working hard' is the subject of 'is difficult'. Whether you use the subject or topic particle depends on context, but if you use either of them you'll probably be understood.

Hiya! I don't know the answer to your question, but if you don't get any answers after a while, you might want to try changing the "base language" to English (since it is primarily written in English) and the "For learning" language to "Korean" (because the question is about the Korean language). I think that way the right people will probably see it and will be able to give you a good answer! :)