The characteristics of written comments on assignments as well as the process of writing them were studied. Interviews to three case-study middle school students, observation of lessons and content analysis of their assignments show that features that promote learning are: comments located next to what needs revision must be reviewed; use of topics to discriminate tasks; vocabulary familiar to students; short and direct comments when involving simple competencies, but supplying strategies when complex competencies are involved. Teachers felt difficulties related to: lack of space to write, selection of what to comment on and, occasionally, avoiding to give part of the answer. Being aware of students' competencies, giving timely feedback, using feedback systematically and student motivation are other factors that determine comments' efficacy. (Contains 1 table.)