Let's start with Moby-Dick (Herman Melville, 1851). Three words:
Call me Ishmael.
Well, that's easy. Or is it? All the diagram does right now is label three separate elements (verb, pronoun, noun) under the heading S (for Sentence). How do the parts work together?
Call is an imperative verb. It does not need a subject, because the subject is understood: something like "[You should] call."
me is the direct object. "You call me." It can be turned into a passive subject, if you prefer (and Melville didn't prefer): "I am called by you."
Ishmael is the trickier part. It is what the Oxford English Grammar calls an "object predicative"; some call it an "object complement." The verb acts on the direct object to turn it into the object predicative. I was just some guy before, but now I become Ishmael.
More complicated sentences will, hopefully, follow.
