Here's why:
* Subject: It lacks a subject, which is the noun or pronoun performing the action. We don't know *who* is hitting the stone.
* Verb: While "hit" is a verb, it's in the imperative form (command). Complete sentences usually have a verb in a different tense.
To make it a complete sentence, you could add a subject:
* "You hit the stone hard."
* "He hit the stone hard."
Or change the verb tense:
* "He hit the stone hard."
* "The hammer hit the stone hard."