I did not thrive at diagramming sentences. I learned enough to fake it, but I could never figure out how the geometric lines on the chalkboard or in my notebook related to what I loved to read or tried to write. But diagramming sentences is part of school, so it must be important. It turns out, not so much. There is no demonstrable link between being able to diagram sentences and being able to write effectively. This fact was codified by the National Council of Teachers of English in 1963, seven years before I was even born.
