To leave a comment on a piece of student work for a student to see, go to the Data page and select Quizzes or Writing from the headers listed. Click the Feedback link below the quiz score:
Type your comment into the field to the right of the student answer you want to leave feedback about. Hit enter to submit your comment.
After you submit your comment, an option to request revisions from the student will appear in the bottom right corner of the quiz.
Note: Comments are private, so only that one student will be able to see the comment you post to their work.