Auxiliary Verb:
* When used with another verb to form a question, negative, or other grammatical structure:
* Can you swim?
* I can't go.
* He can be very stubborn.
Main Verb:
* When it stands alone and expresses ability or possibility:
* I can do it.
* She can speak three languages.
* The dog can sit.
To determine whether "can" is an auxiliary or main verb, look at the structure of the sentence and the meaning it conveys.