could如果是表示“可能”用于一般过去时态时,回答可用could.
e.g.Could he play the piano two years ago?肯定回答：Yes,he could.否定回答：No,he couldn't.
如果表示“委婉”,回答用"can".
e.g.Could I use your computer?肯定回答可用：Yes,you can.但不能用Yes,you could.否定回答可用：Sorry,you can't.不用Sorry,you could't.我想你是问有关这吧.