Which part of speech provides additional details about an action, often ending in -ly?

The correct answer is an adverb. Adverbs are words that modify verbs, adjectives, or other adverbs, providing additional information about how, when, where, or to what extent an action occurs. A common characteristic of many adverbs is that they often end in -ly, such as "quickly," "silently," and "happily." These modifiers enhance the meaning of a verb by specifying the manner in which an action is performed, thus allowing for a clearer and more vivid understanding of the action described.

In contrast, adjectives modify nouns and pronouns, adding details such as size, color, or quality, but they do not typically describe actions. Prepositions, on the other hand, show relationships between nouns or pronouns and other words in a sentence, indicating direction, time, or position, which is different from modifying actions. Conjunctions connect words, phrases, or clauses, serving another function apart from detailing actions. Thus, adverbs uniquely fulfill the role of providing specific details about actions, particularly in relation to the manner of the action.