Quote Originally Posted by keannu View Post
Thanks a lot for your detailed explanation, and Raymott's following example seems to imply it's a one-time experience as the front part back up he couldn't help but do it due to the long preparation and effort.
"I had trouble escaping from prison, but after I had fasted for a month, I could just squeeze through the bars. One time in the past - OK

I really feel ashamed of my numerous expressions of "could" meaning one-time event, so were they all grammatically incorrect(maybe not), but the listeners could understand my expressions(again, could for one-time..)
I've said like these so many times..
"I could meet someone at a specific place at a specific place"
"I could call him at 2pm, I could do it (specific moment)"
I don't think it will be easy for me to convert all of them to "be able to"....
What kind of one time exprience? An exprience of ability or an exprience of ability and doing something? These are different as we see from fivejedjon's post. If you find it hard to convert all of those sentences to "be able to" sentences then perhaps you should ask yourself what you really mean by them. Just ability? Or ability and action together?

I had the same problem with "could" some years ago and yesterday I realized that still I have problems with it (so far I thought that "could" and "be able to" convey the same meaning). At that time I found a solution for it: Whenever I wanted to refer to having the ability to do something and doing it as well in the past I used verbs such as "succeed" and "managed". I will try to find out how people really use "be able to" in such cases.