What is the main function of an adjective in a sentence?

The primary function of an adjective in a sentence is to describe or modify a noun. Adjectives provide additional information about a noun, such as its size, shape, color, quantity, or quality. For example, in the phrase "the red apple," the adjective "red" describes the noun "apple," giving the reader a clearer picture of what type of apple is being referred to.

In contrast, the other choices illustrate different grammatical functions: connecting words pertains to conjunctions, showing action relates to verbs, and replacing nouns involves pronouns. While these elements are crucial in sentence structure, they do not fulfill the role of an adjective, which is specifically to add detail to nouns. Therefore, identifying "describes a noun" as the correct function highlights the specific role that adjectives play in language.