Differences between could and would

   

*Could *

    - Used to express ability or possibility in the past. That is, it has the same meaning as "can" but referring to the past.

    -I could run very fast. I could run really fast. These nuances that before you had the ability to run very fast but now no longer. Therefore, it is a past action.

    -It is also used to express possibility in the future.

     -It could rain tonight. Tonight it might rain. We do not know if it's going to rain, but we think it's possible.

    -Like can, it is used to ask for permission or ask about possibilities through interrogative phrases. The difference with can, is that it could be more formal.Could I ask you something? 
 
* Would *

    - Used to convey a preference as well as to ask about something politely.

    -I would like to go to Paris someday. I would like to go to Paris someday.

    "Ask for something politely."Would you like to go to Paris? Would you like to go to Paris?

IDM