I remember when I was in middle school being tortured by this horrible creature called sentence diagramming. It just made absolutely no sense whatsoever. It seemed pointless and impossible. (My aunt had to do the same thing in the 50's and was left with the same feeling.) Then twenty years later, I was again required to learn it in my graduate school course New Grammars. BING! It suddenly made sense. (Thank you Dr. Schaefer!)
Sentence diagramming is a way to show the relationships between words in a phrase and phrases in a sentence. It is very similar to drawing molecular structures like H2O as H-O-H. I've found it to be an invaluable tool for me as I try to figure out strange and complex yet correct structure and for showing grammatical relationships to students who have an understanding of diagramming. My favorite example of why this is so important came by accident one day at school.
A student who had completed my Advanced Grammar course and had a good understanding of diagramming came to me with a sentence she had written. (She was a strong second-language learner from Macau.) I tried breaking down the sentence for her orally and then I had an epiphany. "Let's diagram your sentence."
Together we sussed out the structure, which had no glaring grammatical errors but was nevertheless, very awkward. Several minutes later, we had a monstrosity of a diagram on the board. Then, we rewrote her sentence and I had her diagram the new sentence. Thirty seconds later, a much simpler, much smaller diagram was on the board. She took one look at the pair and said "A-ha!"
Diagramming is the grammatical equivalent of "A picture is worth a thousand words." But how do you do it? The most well-known method for diagramming sentences is the Reed-Kellogg method, which was developed over a hundred years ago and didn't go out of favor until just after I graduated from school in the early 80's. (Which, coincidentally is about when American English began circling the drain.) When I went back to diagramming about five years ago, I tried and tried and tried to find textbooks or Internet sources that could answer the difficult questions, but I could not find anything. As a result, I may have modified diagramming a little and purists who are more expert than I might cringe, but it works for me and follows the original structures developed by Reed and Kellogg. If anyone can enlighten me or point me to available reference material, I would greatly appreciate it. (However, I do think the original needs updating a bit.)
This is how it works. Take, for instance the following sentence from Great Expectations by Charles Dickens. "I have a recollection of large, unbending women with great noses and rapacious eyes who wore their clothes as though they were armor." This is how the sentence would be diagrammed.
Even with small, almost illegible text in the diagram (Sorry about that), you can see the structure of the sentence. There are three clauses in the sentence. The main clause is an SVO clause with a and a prepositional phrase modifying the object. The object of the preposition, women, is modified by two adjectives and another prepositional phrase. This prepositional phrase, beginning with with, has a compound object, each of which is modified by one adjective.
In addition, women is modified by a subject adjective clause, which also happens to be an SVO clause. The object is modified by the adjective pronoun their. Finally there is an adverb clause, connected by the subordinating conjunction as though, modifying the adjective clause. This final clause is an SVC clause with a noun complement.
This explanation might sound scary, but perhaps I can give an analogy that will ease your fears a little. (Or is it a metaphor? I can never keep that stuff straight. That's why I teach grammar and not literature.)
If you are approaching (or well into, in my case) your middle years, at some point you probably had a distinct fear of computers. I certainly did. I went out of my way to avoid them like the plague. But when I was finally forced to confront my fears and actually buy one of those devil spawn things, I quickly learned a valuable lesson. Computers, like most specialized fields, are cloaked in a veil of in-group vocabulary. Once you learn the basic terms, which doesn't take long, suddenly everything is so much more accessible. Grammar in general, and diagramming in particular, are exactly the same way.
I am very confident that within a short period of time, anyone can learn to diagram complex sentences. I would love to be the one to teach you.