I think the answer is '私は日本語の勉強は楽しいと思います'. '思う' becomes '思い' and 'です' becomes 'ます' because a verb is usually followed by 'ます' but not 'です'. That's why your answer '私は日本語の勉強楽しいと思うです! ' is not grammatically correct. We can also say '私は日本語の勉強は楽しいと思う'. 'です' is eliminated and 'と思う' is added. This is less polite than the first one. But '日本語の勉強は楽しいです！' contains one's thoughts, so '日本語の勉強は楽しいです！' can be translated as 'I think studying Japanese is fun'. '日本語の勉強は楽しい' can be 'Studying Japanese is fun' because this sentence is one in a general sense. If you said to somebody '日本語の勉強は楽しい!' with your eyes bright, it would mean that you tell them your thoughts 'I think studying Japanese is fun!' or 'How fun it is!' etc,. I hope this will be helpful & forgive my English.

★日本語の勉強は楽しいと思います。