Could and can is the same word in different forms, and they are used for different contexts. The word can express the notion of being able to do something. In nearly all cases, it can be replaced with a form of being able to. It can also be employed to ask for permission in certain situations, such as informal ones. Using the word could imply formality. It also sounds more appropriate when addressing someone such as an elder or a boss. “could you please explain that to me again?” sounds a lot better than “can you explain that to me again?” Using words could and can depend on the condition of the situation.