Writing is a developed skill and most people progress in their development if they attempt to solve one problem at a time. If you hand back an exam with one central comment the students are more likely to read it than if you handed them back an exam covered in ink. Unfortunately, early on most schooling teaches students that many marks on a piece of writing mean failure of some kind. So, even when your words are praise, some students may not read them if your words blot out their own words. Ask students why they wrote their exam, sentence, paragraph as they did.
Most writers have reasons for their choices, both the good choices and the bad choices.
Learning the source of the students misunderstanding can be the most efficient road to clarification. Be prepared to refer students to sources of information where they can get help with their writing. Print information about the Writing Center in your course syllabus. Introduce your students to your discipline's style manual when appropriate. Written by Kimberly Town for use in the Ohio State University Writing Center, 1992.