Do prepositions modify verbs, adjectives, and adverbs?
Yes - see below for examples.
Let's test to see if we can!
Let's modify a verb:
He ran in the wrong direction - we modified the verb "ran" to indicate the direction.
Let's modify an adjective:
She was happy when lying on the beach watching the clouds - we modified the adjective "happy" to indicate location.
Let's modify an adverb:
The cat waved it's paw irritably at the feather duster being thrust into its face. - we modified the adverb "irritably" to indicate why it was acting that way (and we also modified the verb "being thrust" to indicate where it was being thrust).