Final answer:
The underlined phrase 'with an axe' in the given sentence 'Then someone chopped it down with an axe' is an adverb phrase.
Step-by-step explanation:
In the given sentence, 'Then someone chopped it down with an axe,' the underlined phrase 'with an axe' is an adverb phrase. Adverb phrases typically modify verbs, adjectives, or other adverbs. In this case, the adverb phrase 'with an axe' specifies how the action of chopping down was performed.