When you want to say a question , you can say "열심히 공부하세요? " However if you want to say it as encourage , " 공부 열심히 하세요 ". actually, "열심히 공부하세요" and " 공부 열심히 하세요 " are same mean . but It's different if you use a questions marks "? " .

Never mind. I guess everyone has contradicting opinions of what is right and wrong.