A can与 be able
1 shall/will be able是唯一的将来时形式：
Our baby will be able to walk in a few weeks.
我们的宝宝再过几个星期就能走路了。
2 can和be able都可用于现在时，can则更常用些：
Can you/Are you able to type?
你会打字吗?
I cant pay you today.Can you wait till tomorrow?
今天我不能付你钱。你可以等到明天吗?
Could you wait?
您能等候吗?
3 在现在完成时中，必须用be able形式：
Since his accident he hasnt been able to leave the house.
自从他遇到意外以后，他一直没能出门。
B could
1 如存在着条件的含义，could可以用来表示现在时的意义：
Could you run the business by yourself?
你能自己管理这企业吗?(如果必要的话)
Could he get another job?
他能找到另一份工作吗?(如果他辞掉这份工作的话)
I could get you a copy.
我可以给你弄到一本。(如果你想要的话)
在头两个例句中could可以用would be able替代。
2 could you?是一种极好的表示请求的方式。它跟 would you?
可以替换使用，但比后者更有礼貌些：
Could you show me the way/lend me￡5/wait half an hour?
你能给我指一下路/借我五英镑/等半小时吗?
Could you please send me an application form?
您能否寄给我一份申请表?
couldnt you?也是很有用的：
Householder：Could you come and mend a leak in a pipe?
房主：你能不能来修理一下管道?
Plumber：Would sometime next month suit you?
管道工：您看下个月什么时候合适吗?
Householder：Couldnt you come a little earlier?
房主：你是否能提前一些日子来?
C could和was/were able用于表示过去的能力
1 只表示一般的能力时，两者都可用：
When I was young I could/was able to climb any tree in the forest.
我年轻的时候，林子里的什么树我都能爬。
2 表示能力+特定行为时，要用 was/were able：
Although the pilot was badly hurt he was able to explain what had hap- pened.
飞行员的伤势虽然很重，但是他还能够解释发生的一切。(他能够而且的确作了解释。)
The boat capsized quite near the bank so the children were able to swim to safety.
船是在离岸很近处翻的，因此，那些孩子能够游到安全的地方。(他们能够也的确游到安全的地方。)
但是，这一规则用于否定句中表示未能发生的事时或与表示感觉的动词连用时，就不那么严格了：
He read the message but he couldnt/wasnt able to understand it.
他读了那张纸条，但却不懂是什么意思。
I could/was able to see him through the window.
我透过窗户能看见他。
D 过去完成式是 had been able：
He said he had lost his passport and hadnt been able to leave the country.