能力
Could 和 can 皆可表示能力 (意為「能，會」)，前者表示過去的能力，後者表示現在和未來的能力。在此用法中，could 為 can 的過去式。例如：
- When I was young, I could play the piano, violin, and cello. (年輕時我會彈鋼琴、拉小提琴和大提琴)
- He can speak English, French, German, and Spanish. (他會講英文、法文、德文和西班牙文)
- I can walk home after school tomorrow. (明天放學後我能走路回家)
當 could 用作 can 的過去式來表示過去有能力做某事時，這是指過去某一段時間發生的事實。對於過去有能力去做某一偶發的單一事件而且做了，在肯定句中我們就不能使用 could，必須使用 was/were able to 或 managed to 或比較正式的 succeeded in (後接動名詞)，意為「成功做到，順利完成」。例如：
- I was able to get the concert tickets I wanted. (我 [成功地] 買到了我想要的演唱會門票) (正)
= I managed to get the concert tickets I wanted. (正)
= I succeeded in getting the concert tickets I wanted. (正)
I could get the concert tickets I wanted. (誤) - We were able to find the missing girl we were looking for. (我們 [成功地] 找到了我們要找的那個失蹤女孩) (正)
= We managed to find the missing girl we were looking for. (正)
= We succeeded in finding the missing girl we were looking for. (正)
We could find the missing girl we were looking for. (誤)