Final answer:
In the sentence 'The puppy trotted sadly,' 'sadly' is the adverb that modifies the verb 'trotted,' describing the manner of the action.
Step-by-step explanation:
An adverb is a part of speech that modifies verbs, adjectives, or other adverbs. It often describes the manner, time, place, or reason related to the action or state the verb expresses. In the sentence 'The puppy trotted sadly,' the word 'sadly' is the adverb. It modifies the verb 'trotted,' describing the manner in which the puppy trotted.
Adverbs can often be recognized by their '-ly' suffix, though there are adverbs that do not follow this pattern. Just as adjectives modify nouns, adverbs provide more detail and nuance to verbs and sometimes to adjectives and other adverbs. Additionally, verbs are action words or state-of-being words that require a subject and sometimes an object. An adverb can also qualify or modify an adjective, providing additional detail to the description.