Culturally, you should maintain respect when talking to your professor. If he or she made a mistake, and you wish to correct him or her, utilize a high-context approach: during his or her office hours or by e-mail, ask your professor to clarify which way is correct for you since you had learned one thing, but the professor used a different way. You could ask him or her if that is a dialect of standard English.
Your professor will quickly realize that you are allowing him or her to save face and should thank you for pointing out the linguistic mishap. Later, you may find that your politeness may have been in fact rewarded.
Eric Paul Monroe