In the phrase "the tall building," what part of speech is "tall"?

In the phrase "the tall building," the word "tall" functions as an adjective. Adjectives are words that describe or modify nouns, providing more detail about them. In this case, "tall" describes the noun "building" by indicating its height. This descriptive role is essential to understanding the characteristics of the noun it modifies, helping the reader visualize or comprehend the specific type of building being referred to.

The other parts of speech—including nouns, verbs, and adverbs—serve different grammatical functions and do not apply here. Nouns represent people, places, things, or ideas; verbs express actions or states of being; and adverbs modify verbs, adjectives, or other adverbs, often indicating manner, place, time, or degree. Since "tall" is indeed describing a quality of "building," it is categorized correctly as an adjective.