i could swim，過(guò)去發(fā)生的 反復(fù)發(fā)生的能力
i could have done this before. 推測(cè)的能力，針對(duì)具體的某件事情的能力。
說(shuō)到 could 在描述過(guò)去的事情的時(shí)候 一定是 他做了（he could and he truely made it） could have 是表示猜測(cè)他可能做了 至于做沒(méi)做并不確定。
you know what, when she needed money so badly she could work in KFC. 這是說(shuō)話人親眼見(jiàn)到了事實(shí) 就會(huì)這樣說(shuō) 后面沒(méi)說(shuō)完的話應(yīng)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(shuō)話人在猜測(cè) 可能性，真實(shí)做沒(méi)做，并不知道。