A.    肯定/否定句*

I can swim now. (現在式)

She cannot write well. (現在式) 

He could swim when he was five. (過去式)

They couldn’t catch the bus yesterday morning. (過去式) 

You are not able to perform on stage. (現在式)

He was not able to use the computer when he was six. (過去式)

We want to be able to solve the problem quick. (不定詞的)

*注意! 

1)  Can/Cannot, be able to用在現在式

2)  Could/Couldn’t, was/were able to用在過去式;

  表達過去的能力,可用could, was/were able to:

Jane could/was able to write when she was four.

表達過去一特定時期所有的成就,僅用was/were able to:

Jane was able to win the swimming contest two years ago.

3)  若句子所表達為非過去或現在式的其他句子,則用be able to:

They will not be able to make it on time. (未來式)

B.     問句與回答

Can you type fast?  Yes, I can./No, I can’t.

Could he wipe all the windows? Yes, he can./No, he can’t.

How well can she perform on the stage?

How high could they jump?

Are you able to solve all the math problems?  Yes, I am./No, I’m not.

Is he able to run fast?  Yes, he is./No, he isn’t.

How deep are they able to dive into the ocean?