语法:can与be able to的比较

…but I was able to provide a translation service so the Vietnamese could gain business permits. (Passage A, Unit 9)

上句中用了情态动词be able to，表示能力。 “I was able to provide a translation service”意思是“我（成功地）向越南人提供了翻译服务”。
情态动词can与be able to都可以表示能力。两者的用法和意义有时比较接近，有时却有较大的区别。
1．如果用于现在时，can通常表示现在的能力或不受时间限制的能力。例如：
I can do the job. Please let me do it.　（现在的能力）
Can you read the letter to me?　（现在的能力）
He can swim.　（不受时间限制的能力）
You can certainly cook, even if you can’t do anything else.　（不受时间限制的能力）

在这种情况下，也可用be able to，但两者相比，can更常用。例如：
I am able to do the job.
He is able to swim.
You are certainly able to cook, even if you aren’t able to do anything else.

2．如果表示将来的能力，通常用will / shall be able to。例如：
The little girl will be able to run very fast.
He will be able to pass the test.
I’ll be able to work out that problem after a good sleep.
One day he will be able to win the Nobel Prize.

但如果将来的能力是在现在为将来所做的计划或安排中表达出来的，则常用can。例如：
We can discuss that later.
I’m too busy today, but I can come to your place tomorrow.
He will be free tomorrow afternoon, so he can go to the lecture.
Can you come to a party on Saturday night?

3．如果表示过去的能力，通常用could或was / were able to。在表示过去的一般能力时，两者没有很大差别。例如：
She could / was able to sing well when she was a little girl.
My mother could / was able to speak four languages.
I could / was able to play the piano when I was young.
Jack could / was able to dance, but he can’t now.

但如果要表示“一时的能力”，即“在过去某时有某种能力，并顺利地完成某事”，则要用was / were able to，而不能用could。例如：
After hard work, he was able to win the prize.
[误] After hard work, he could win the prize.
To my surprise, they were able to finish the task in time.
[误] To my surprise, they could finish the task in time.
Though he was sick, he was able to swim across the river.
[误] Though he was sick, he could cross the river.
After five hours of climbing, we were able to reach the top of the mountain.
[误] After five hours of climbing, we could reach the top of the mountain.

此外，was / were able to经常可用managed to 或succeeded in (doing)代替。例如：
After hard work, he managed to win the prize.
To my surprise, they succeeded in finishing the task in time.
Though he was sick, he managed to swim across the river.
After five hours of climbing, we succeeded in reaching the top of the mountain.

在否定句中，无论是表示过去一般的能力还是一时的能力，都用couldn’t / could not。例如：　
I couldn’t decide what to do when I was a young student. (一般的能力)
She could speak well, but she couldn’t write a single word.　（一般的能力）
Though he tried hard, he couldn’t pass the exam.　（一时的能力）
Simon was so drunk that he couldn’t find his home.（一时的能力）