i could swim，过去发生的 反复发生的`能力
i could have done this before. 推测的能力，针对具体的某件事情的能力。
说到 could 在描述过去的事情的时候 一定是 他做了（he could and he truely made it） could have 是表示猜测他可能做了 至于做没做并不确定。
you know what, when she needed money so badly she could work in KFC. 这是说话人亲眼见到了事实 就会这样说 后面没说完的话应该就是 and i saw it.
you know what, when she needed money so badly she could have worked in KFC.
这是说话人在猜测 可能性，真实做没做，并不知道。