If you want to leave simple feedback for your students, you can leave a text comment directly from the Gradebook.
Note: You can also grade assignments from the comments window in the Gradebook. However, you cannot add a comment and assign a grade at the same time.
Open Comment Window
Locate the student and assignment. Hover over the grade cell and click the Discussion icon.
Type your comment in the add a comment field  then click the Post Comment button .
Where there is more than one comment for an assignment, comments will be listed in chronological order with the oldest comments appearing at the top and the newer comments appearing near the bottom.
After you have posted your comment, the comment window will close automatically.
If you want to view the assignment in SpeedGrader, or if you want to remove your comment, open the comment window again and click the More details in the SpeedGrader link.