1．表示能力
(1)表示现在的能力
(2)表示将来的能力：will be able to
(3)表示过去的能力
The fire spread through the hotel very quickly but everyone was able to get out.
尽管这场大火迅速蔓延到了这个宾馆，但是每个人都逃了出去。(表示过去有能力做并且成功地做了)
I could have worked out the problem, but I was too nervous.
我本来可以解出这道题，但我太紧张了。(表示本来有能力做但未做)
