“ With a little more effort he could have earned a B.” Better,
“ With a little more he effort he would have had greater understanding of decimals.”
The goals for our students are that they are risk-takers, effective communicators, problem solvers, persistent, team players, independent learners, and so on. Comments are more powerful when geared toward these goals, not necessarily toward higher grades.
It is nearly impossible for a struggling kid to know what “try harder” or “a little more effort” really means. “Turn in work on time” or “participate more in class by asking thoughtful questions” are both more specific.
Any behavior you’d like to see more of; anything a student is beginning to do which is an improvement - try to reinforce that behavior. Best to say something that a student might not even be aware they are doing.