As a student myself, I struggled with diagramming in junior high and college (we never saw diagramming in high school). As a student teacher trying to help a young girl edit an essay, I pointed out a faulty sentence, but was unsure why the sentence didn't make sense. The student immediately began to diagram the sentence to find her error! After that, I was hooked!
I taught junior high language arts for 10 years. My students diagrammed sentences every day. As a result, they not only learned how words functioned in sentences, but they were better able to correct grammatical errors in their own writing and knew how to use commas correctly. They were also able to construct sentences of varied length and style without constantly being tripped up by dreaded run-on sentences (which, by the way, they were able to find and correct on their own).
My own daughter learned to diagram in eighth grade. She knows grammar very well, and says that the diagramming of sentences is what helped her. She is about to graduate high school and has taken Latin for all four years. She says her knowledge of grammar, learned through diagramming, was a tremendous help to her study of Latin.
Now I am a graduate student preparing my proposal for action research to be conducted in the fall. My research will seek to answer the very question Susannah has asked - Does sentence diagramming help students? I am looking for any related research on sentence diagramming (which is what brought me to this site). If there is anyone out there who knows of research, please let me know.
weiglea

centenarycollege.edu
Thanks!
Susannah - Please remember that diagramming is not an end in itself, but merely a tool for the study of grammar. When I taught language arts, I never graded my students on diagramming.