What part of speech is the word harder?

"Harder" can be two parts of speech, depending on how it's used:

* Adjective: When describing a noun, "harder" is an adjective. For example: "This rock is harder than that one."

* Adverb: When modifying a verb, adjective, or another adverb, "harder" is an adverb. For example: "He studied harder for the exam."

To determine which part of speech it is, consider the word it modifies or the context in which it's used.