How do I diagram this sentence ("Boats docked at Toronto in Ontario should be registered with the town hall.")?
The phrase, "docked at Toronto in Ontario," is a past participial phrase. That phrase is adjectival in nature. It modifies the head of the subject, "boats."
The phrase, "should be registered," is a verb phrase. "Should" and "be" are auxiliary verbs, with "should" being a modal auxiliary verb (@user405662). "Registered" is the main verb.
The phrase, "with the town hall," is a prepositional phrase. It acts as an adverbial phrase, and modifies the main verb, "registered."