can,could vs be able to (Ability)

1. can 既表示现的“能力”，也能表示将来的“能力”。但表示现在的能力通常用can，表示将来的能力，一般用will/shall be able to。

I can’t do it now, but I can do it later.

Fire cannot destroy gold.

I'll be able to speak German in another two years．

2. be able to可以用于各种时态, 也可用于情态动词后：is/ are able to, was/ were able to, will / shall be ablte to , ought to be able to, might be able to.

They will be able to tell you the news soon. 他很快就能告诉你消息了。
I shan’t be able to come on Janury 1.

I haven’t been able to get in touch with her.

I ought to be able to live on my salary.

4. could表示过去经常性或一般性的能力.

My father could speak ten languages．

At that time I could still read without spectacles.

5. 表示过去一次性的或经过一番努力, 终于能够成功地完成某一具体动作时, 只能用was/were able to, 不能用could。

He was able to flee Europe before the war broke out.
= He managed to flee Europe before the war broke out.

= He succeeded in fleeing Europe before the war broke out.

I was able to finish my homework in an hour．

在否定句中没有此限制：

I couldn’t / wasn’t able to play the piano when I was at school.

He got so drunk that he couldn’t / wasn’t able to find the front door.

6. can/ could 的主语既可是人，又可是物； be able to 只用于有生命的名词、代词作主语的句子中。

It can be referred that…

参考资料：

[1]  薄冰，何政安. 薄冰英语语法[M]. 北京：开明出版社，1998.

[2]  L.G. 亚历山大. 朗文英语语法[M]. 北京：外语教学与研究出版社，1991.

[3] （英）汤姆逊，（英）马丁内特文. 牛津实用英语语法（第四版） [M]. 北京：外语教学与研究出版社，1999.

[4]  张道真. 现代英语用法词典（重排版）[M]. 北京：外语教学与研究出版社，2005. 

[5]  张道真. 英语语法大全（上）[M]. 北京：首都师范大学出版社，2008.

[6]  张道真. 英语语法大全（下）[M]. 北京：首都师范大学出版社，2008.

[7]  张振邦. 新编英语语法教程（第五版学生用书）[M]. 上海：上海外语教育出版社，2009.

[8]  赵振才. 英语常见问题解答大词典（增订版第四版）[M]. 西安：世界 出版西安公司，2009.