"Can" and "Could" are modal verbs in English grammar used to express ability, possibility, permission, and to make requests or offers. Their usage in the United States follows the general rules applicable in English, but with some nuances specific to American English.
"Can":
"Could":
In the U.S.:
"Can" is common in both casual and formal speech. "Could" is preferred for politeness or formal situations. Both are widely understood and used, with "can" often for permission in casual settings.