Yes, many of us have never diagrammed a sentence since high school. What was the purpose? Diagramming happened so long ago that you may not even remember or maybe you did not recognize the evolution in your writing that resulted. However, you are questioning passing on this skill? Right? I knew you were. I figured that would have something to do with you continuing to read.
Ultimately, the most valuable part of diagramming comes from using it alongside teaching the parts of speech. Identification is so much easier. You must point out that a sentence, while a complete thought, must contain a subject and predicate. Diagramming illustrates the parts of speech, subject and verb.
Diagramming also demonstrates how the parts of a sentence function and the relationships. A compound sentence is best understood by creating a diagram of the sentence. You can visually see the joining of subjects and verbs. This is easily extended to understand verbal nouns, prepositional phrases, various clauses, etc.
Think of diagramming much in the same was as you would an abacus or blocks for math. It is a tool used to teach grammar. Diagramming is an important part of teaching sentence construction too. It is especially important and useful for a visual or kinesthetic learner. As a matter of fact, a program similar to Winston Grammar or one laden with diagramming like Rod & Staff would be essential for teaching grammar for these types of learners.
Diagramming is only a tool for grammar. It is not the point of the focus. Grammar is the focus. Correct grammar is the point of studying.
As well, don’t confuse writing tools such as narration, copy-work, imitation, and sentence combining with diagramming. Tools for writing are not the same as tools for grammar. While they may illustrate grammar in action, they do not show sentence construction in terms of grammar but rather stylistically.
SenDraw can be used to create diagrams for sentences that you have already analyzed. SenDraw will not automatically analyze and diagram sentences for you.
This program allows students to manipulate parts of a sentence into a standard Kellog-Reed sentence diagram. There are four levels: Simple sentences, compound/complex sentences, infinitival/that-clause sentences, and miscellaneous sentences. The student uses a mouse to drag sentence elements into their places in the diagrams.
Higher Lessons in English (Google Books)
Sentence Analysis by Diagram (Google Books)
Workbook and Text are available.
Grammar By Diagram