Two points in response to Robert’s post.

First, I don’t think that being taught to diagram sentences is the only way one can learn how to use English well. In the old days (I was learning this stuff ca. 1960), diagramming was just one part of a whole package of English language work that included vocabulary, spelling, punctuation, paragraphing, book reports, and other things. If diagramming had not been in that mix, would it have mattered much? Maybe not, but it fit in with the big picture of teaching youngsters how English works.

Second, I am not convinced that there is a language instinct. What makes me doubt that is the atrocious, scatterbrained writing I used to see from many of the college students I had. Whatever instinct there may be, it needs a great deal of refinement through the kind of work mentioned above. Few young people now get much of that work.