It's not easy to clarify it without definite samples. We can use them interchangeably. It's more important to learn when and how it can be used in colloquial language rather than just what they mean. SEN KÖTÜSÜN! - That means you're evil-minded or something like shows offensiveness. Çok kötüsün :) or Çok fenasın :) - It usually happens while partners are on the phone chatting with each other. Or it could be a someone's subtle idea that you're more or less impressed. It's usually followed by courtship or grinning. :D Fena - That word could refer to something good or bad. For example: Sınav çok fenaydı - Exam is bad so I sucked. Fenerbahçe çok fena oynadı - Fenerbahçe played well, so they won. So, you'd better go through the words you learn with their context separately. Don't tend to generalize them with a strict form. It sometimes can look like hard. But, It's what it's.