What part of speech is 'quickly' in the sentence "He runs quickly"?

In the sentence "He runs quickly," the word 'quickly' functions as an adverb. Adverbs modify verbs, adjectives, or other adverbs, and they often describe how, when, where, or to what extent something occurs. Here, 'quickly' describes the manner in which 'he runs', indicating that his running is done in a fast manner.

The adverbial role is clear because it answers the question "How does he run?" with 'quickly'. This helps convey the speed of the action in a direct, descriptive way, enhancing the overall meaning of the sentence. The other parts of speech listed, such as nouns, verbs, and adjectives, do not fit this context since they serve different grammatical functions that do not apply to the word 'quickly' in this sentence.