Yes, "could" is the past tense of "can". It's not possible to use "can" in past tense. However, it is possible to use "could" in present/future sense to signify an unsure or unlikely situation. "You could be right" means you may be right, but I can't be certain. You would use "could" when hypothesizing, such as in the Second Conditional: "If we called him, he could help us." Also, "Could you help me?" is recognised as a polite request - you don't wish to put any pressure on the listener.

Can/can't is present tense, whereas could/couldn't is past tense. Examples... She can't answer the phone right now. She couldn't answer the phone yesterday.

I think can/can't used in present. examples I can sing well.But I can't dance. could/couldn't used in past tense. examples I could go to party last night. I couldn't go to class.