Intransitive verbs

Intransitive verbs appear in complete sentences that are formed in this way:
(i) Subject verb "He looked" or
(ii) Subject verb preposition noun "He looked at the cat."

• Ouch! You trod on my toe!

{on my toe} adverbial prepositional phrase modifying tread.

There is no object in the sentence: Ouch! You trod {on my toe!} - Compare: "He trod silently."