i could swim，過去發生的 反復發生的能力
i could have done this before. 推測的能力，針對具體的某件事情的能力。
說到 could 在描述過去的事情的時候 一定是 他做了（he could and he truely made it） could have 是表示猜測他可能做了 至于做沒做并不確定。
you know what, when she needed money so badly she could work in KFC. 這是說話人親眼見到了事實 就會這樣說 后面沒說完的話應該就是 and i saw it.
you know what, when she needed money so badly she could have worked in KFC.
這是說話人在猜測 可能性，真實做沒做，并不知道。