Here's why:
* Subjects are nouns or pronouns. "Can" is a helping verb (also called an auxiliary verb). It's part of a verb phrase and helps to express tense, mood, or voice.
* Subjects perform the action of the verb. "Can" doesn't perform an action. It modifies the main verb to indicate ability or possibility.
Example:
* Incorrect: Can go to the store.
* Correct: I can go to the store. (Here, "I" is the subject)
To use "can" in a sentence, you need a subject that performs the action:
* She can sing.
* The dog can fetch.
* We can learn.