Here's why:
* Predicate Adjective: A predicate adjective modifies (describes) the subject of the sentence. It usually follows a linking verb (like "feel," "seem," "become," "appear," etc.).
* Noun: A noun is a word that names a person, place, thing, or idea.
In the sentence "I feel lonely," "lonely" describes the state of the subject "I," and it follows the linking verb "feel." Therefore, it functions as a predicate adjective.