Can students ask for changing grades? If a student feels that the grade he received is lower than he expected, because he has been working very hard, and truly trying to understand things, but sometimes at the cost of sacrificing details. But the instructor might think differently, as the student sometimes made careless mistakes in assignments and didn't have much time doing them.

Can he email the instructor for possibility of changing his grade, stating the reasons above? He knows if he never asks, the grade will stay the same, so he wants to have a try. Also as far as he knows, course grading generally has more or less flexibility.

If he does, is there some ethic issue that the student should be aware of? 

Thanks!

PS: This is in a U.S. university.
A student sure can ask for re-grading, but he or she should have strong evidence that what was graded was better than perceived by the grader. It is a very bad idea to try argue how the teacher should be grading (e.g., asking her or him to grade according to what work you claimed you provided rather than the quality of your homework or exam). These questions are up to the teaching teams, not up to the students, and yes that makes the situation rather unbalanced. What you can do about this is to ask for a general change in suitable meetings if they exist.

Moreover, I would like to point out two things :


most of the time, grading is not for acknowledging good effort, but to measure if the student understood enough of the class to have a decent chance in the next one (at least in fields where classes depend strongly one on another, like in mathematics);
my experience tells me that students that consider they understand well but do poor exams in fact understand much more shallowly than they think, and their grades should be taken as an indication that the way they work is probably not good enough.