I do not understand that in which place Could will be used & in which place would will be used.?

But I would suppose that “would” is more polite, because it expresses the idea of probability, and of willingness, and of the desire that something be done, whereas “could” is more in the realm of ability (yes I can). And according to the American Heritage Dictionary, “would” is used to make a polite request

Only past tense and also in polite form like this: Could you tell me the reasons? Would you take some more...?