When asking a question (ex: Can/Could you help me?) there isn't really a difference. Could is slightly more formal but Can is perfectly fine as well in many situations. In a sentence like "I can/could do it". Could is making the statement hypothetical, the person thinks they can do it. If you say Can here, you are much more certain of yourself. Lastly in "I can't/couldn't do it", Can't is used to talk about the present moment, right now. Couldn't is referring to the past, before right now. You can use Can't to talk about the past as well, many English speakers find this normal. The only way to fully understand Could is to hear it used in context a lot but then again it isn't a crucially important word in English, just something to maybe change your tone or better your manner of speaking.