He　has been able to finish the work on time　。(不可用can)

After years of hard work he　was able to win the prize 。(不可以用could)

He　could't climb the mountain 。(没有能力爬，因而也没有爬)

He　was not able to climb the mountain 。(尝试爬过，但是没能爬上去)

另外：

①be able to 强调能力，通常表示一次性的事件，一般不表示经常性的事件。

100. He insisted that we all ___ in his office at one o’clock. A. be B. to be C. would be D. shall be

99. I wish that you ___ such a bad headache because I’m sure that you would have enjoyed the concert. A. hadn’t B. didn’t have had C. hadn’t had D. hadn’t have

98. I do not have a job. I would find one but I ___ no time. A. had B. didn’t have C. had had D. have

97. I left very early last night, but I wish I ___ so early. A. didn’t B. hadn’t left C. haven’t left D. couldn’t leave

96. The librarian insists that John ___ no more books from the library before he returns all the books he had borrowed. A. will take B. took C. take D. takes

95. I wish I ___ a young man again. A. am B. was C. were D. will be

94. If I ___ the money, I would have bought a much bigger car. A. possessed B. owned C. had D. had had

93. He has just arrived, but he talks as if he ___ all about that. A. know B. knows C. known D. knew

92. Had I known her name, ___. A. or does she know mine? B. and where does she live? C. she would be beautiful D. I would have invited her to lunch