Good feedback improves student learning. It has the following qualities:

Specific: It is a tool for future change.  Ask yourself, "What worked?" or "What does the student understand?" Then ask, "What needs improvement?"  

Actionable: Emphasize what could be done differently rather than what is wrong.  Actionable feedback is often in the form of a question.  "How could you have justified this differently?"

Timely: The most effective feedback is immediate and frequent.  How can your feedback be timely for both formative and summative work?

Respectful:  Make an effort to look for the good while still focusing on future changes. How can this work be an asset for future learning not just for this student but other students?