Conscientious teaching requires marking all grammar and language errors.
Look over a set of papers quickly and return error-laden essays for proofreading and correction. Students usually pay attention to comments only when they are given a chance to revise. It makes more sense for you to invest your time and energy responding to the first draft and to make these comments truly facilitative. Respond to the final draft only briefly, and let these be evaluative. More is better in terms of how much you respond to the problems in the paper Students are often overwhelmed and paralyzed when they receive essays on which the instructor's comments trail into every margin and leave a depressing map of error and negative response.