could swim well. ---- could is the past tense of can , knowing how to do something . eg He could swim well last year but he can not do it well now . was able to swim well. --- able is the ABILITY TO DO something , it means < in the past , he has the ability to swim well eg He was able ( to have the ablility ) to swim across the river because he could ( know how to ) swim well I dont think you are not able to finish this beer because you cannot dink beer fast . I can do it because im able = im able because i can , can you see the difference

Hello Keiko, They are both referring to an ability in the past so both are possible and correct - there is no difference in meaning in this case. Hope this helps Bob

Chris, thank you very much for your help again. It is very kind of you to send very helpful message. I will review to know the difference. I know it is almost the same, but my grammar book written in Japanese say their meaning is different. I will read your message again and again until understand. from keiko

I'm with Bob - they are the same. Kenny's answer is confusing, especially so for a non-native English speaker. Here 'could' means 'was/were able to'.