Here's why:
* Subject: The subject is the person, place, or thing that performs the action of the verb.
* Verb: The verb describes the action or state of being of the subject.
Examples:
* The cat slept. (Subject: "cat," Verb: "slept")
* She is running. (Subject: "she," Verb: "is running")
* We will eat pizza. (Subject: "we," Verb: "will eat")
Without both a subject and a verb, you have a phrase, not a clause.