In the example "The blue shirt," what is the adjective?

In the phrase "The blue shirt," the adjective is "blue." Adjectives describe or modify nouns, providing additional information about them. In this case, "blue" is giving us more detail about the noun "shirt," indicating its color. Adjectives can enhance understanding by adding characteristics to a noun—here, specifying that the shirt is of a particular color.

The other components of the phrase serve different grammatical purposes. "The" functions as a definite article, helping define the noun but not describing it. "Shirt" is the noun being described, while "is" is a verb that serves no descriptive function in this context. Thus, "blue" is clearly the word in the phrase that acts as an adjective.