Dave Barry is one of my all-time favorite humorists. Here’s his take on how to diagram a sentence.
Ask Mr. Language Person
Q. Please explain how to diagram a sentence.
A. First spread the sentence out on a clean, flat surface, such as an ironing board. Then, using a sharp pencil or X-Acto knife, locate the “predicate,” which indicates where the action has taken place and is usually located directly behind the gills. For example, in the sentence: “LaMont never would of bit a forest ranger,” the action probably took place in a forest. Thus your diagram would be shaped like a little tree with branches sticking out of it to indicate the locations of the various particles of speech, such as your gerunds, proverbs, adjutants, etc.
Dave Barry
Thanks to The Passive Voice.
