Or at least to the English subsection--diagramming sentences! Yes, it's true! Klaus raised his score on the English subsection by 10 whole points (from 59th to 96th percentile) by taking a half hour to learn to diagram sentences. I couldn't believe he'd gotten all the way through the first half of 10th grade Enriched English without learning to identify the subject and predicate of a sentence. No wonder he had so much trouble last year. How would he correctly identify subject-verb agreement problems if he couldn't identify the correct subject and verb?
As you might expect, this method of teaching grammar/parts of speech works particularly well with visual-spatial learners. Plus, it's loads of fun, kind of like cracking a code. With Klaus, I used exercises from the following sites:
From the Student Learning Assistance Center of the Alamo Community College District--very clear, step-by-step instructions for basic diagramming. This is primarily what Klaus used.
AP Language and Composition Resource Page from Southwest High School's Language Arts Department, gets into a little more detail on the complexities.
So when would someone need to know how to do this? Maybe never, unless he or she wanted to rock the house on the college entrance exams or in a college composition class. I used my ability to deconstruct a sentence when I was doing freelance paper-editing in college. It's not being able to draw the diagram in the right way that's important, but rather being able to analyze the structure of a sentence, brick by brick, and see the relationships between the words, phrases and clauses. That's when writing begins to become the tool, rather than the taskmaster.