the difference between "can and could" is between ability vs. possibility. I *can* walk to the door means that I am able to. My legs are fine, there's nothing blocking my way, I can easily walk to the door. I *could* walk to the door means that I may be able to if I either want to or are able to. I *could* walk to the door if there wasn't a large hole in the floor. I *could* walk to the door if my legs weren't broken. It depends on what you are trying to tell the person :)

We use the modal can to make general statements about what is possible: . . We use could as the past tense of can: