Here's why:
* Subject: A sentence needs a subject, which is the person, place, or thing performing the action. "Can sit" is missing a subject.
* Verb: "Sit" is a verb, which describes the action.
* Complete Thought: The sentence doesn't express a complete thought.
To make it a complete sentence, you need to add a subject:
* "I can sit."
* "The dog can sit."
* "The chair can sit 2 people."
Let me know if you'd like to explore other parts of a sentence!