Making comments in the margin of the paper (whether written or digital) is an important part of engaging in a dialogue with the student about their ideas. When you make a margin comment, you want to have a specific purpose in mind rather than writing as a reaction. You should also be clear with the students on how to "read" your comments: should they be used to help with a future revision or to assist with the next writing project?
Here are the different types of margin comments that you will want to use given your purposes:
Directive: These comments provide specific direction on ideas and/or writing. You should be specific with these comments about a) what/where you see the problem and b) a specific solution on how to fix it. For instance, if a student should move certain ideas from one paragraph to another, a directive comment would tell them what information should be moved, why it should be moved, and where specifically to move it to.
Conversational: These comments engage in conversation with the ideas by asking questions, pointing to additional lines of inquiry, and/or offering an alternative approach. For instance, you might suggest that a student looks into another aspect of the debate s/he is covering to help contextualize their thesis.
Corrective: These comments are aimed at correcting specific elements of the writing itself, such as pointing out a misused punctuation mark or a grammatical mistake. When making corrective comments, you should be a) specific with your comment, b) provide a model correction, and c) use consistent marks.
Evaluative: These comments help faculty to link their observations with the specific grade they assign to the essay. These comments provide students with insights into your standards and expectations for their work and help to apply the information presented in the prompt and rubric. Evaluative comments should not be used to "justify" or "defend" a grade, but to explain it.
The following best practices apply to all of these different types of margin comments: