Can an adverb only modify verbs?

No, adverbs can modify more than just verbs. While they are most commonly used to modify verbs, adverbs can also modify:

* Adjectives: *extremely* cold

* Other adverbs: *very* slowly

* Phrases: *unfortunately* for him

* Clauses: *happily* ever after

* Sentences: *Frankly*, my dear, I don't give a damn.

So, adverbs are versatile words that can modify a variety of grammatical elements to provide more information about them.