Which part of speech answers the questions "who" or "what" in a sentence?

The part of speech that answers the questions "who" or "what" in a sentence is the subject. The subject of a sentence is the noun (or pronoun) that performs the action of the verb or is described by the verb. In simpler terms, it identifies the person, place, thing, or idea that the sentence is about.

For example, in the sentence "The dog barks," "the dog" is the subject because it answers the question "who is barking?" Similarly, in "The book is on the table," "the book" answers the question "what is on the table?" This highlights the essential role of the subject in constructing meaning in sentences. The other parts of speech mentioned—such as objects, verbs, and adverbs—serve different functions and do not specifically answer the "who" or "what" questions as the subject does.