Although there are many cases where a particular usage is unambiguously located at one extreme on such a scale, other uses fall in the ambiguous middle. Rules are what some linguists study, but many writers are in fact unaware of most of them. We want students to learn to control their rhetorical choices, including those that are prescribed by standard English. To exercise such control, students need, of course, to know what their choices are. Teaching grammar as an abstract system of principles may help some student writers, but its effect on others can be devastating, depending, of course, on how it is taught and what the students need to know.
Correct usage is learned in many ways.
Students of a foreign language are able to learn it, in part, through memorization of rules. Most of us learned the grammar that we use in other ways, and this is the case for most of our students too. Teachers must understand basic grammatical elements in order to assess students' writing problems and help them overcome their difficulties. Naming the problem makes a vaguely-sensed difficulty visible and more remediable. . . . If the teacher cannot answer student questions about me vs. I or who vs. whom .