1：表示“能力”
表示能力，可用 can , could ,be able to .can 既能表示现在的“能力”，也能表示将来的“能力”。
eg:
Look ! i can do it .
I can't do it now ,but i can do it later.
用can 表示现在的“能力”，即可表示做某桩具体事情的“能力”
eg:
I can climb this cliff.
Is there anything i can do for you ?
也可泛指一般的“能力”
eg:
He can drive a car .
He is only four ,but he can read.
She can't play the piano.
Fire can not destroy gold.
can表示“能力”与be able to 同义，在许多情况下可以交替使用。
eg:
As i have got enough money, i can /am able to help her .
尤其是将来的“能力”，通常用will/shall be able to 表示。
eg:
表示过去的“能力”，可用could和was /were able to ,但在肯定句中，could 所表示的“能力”仅是泛指过去的一般能力。
eg:
She could play the piano when she was only six.
At that time i could still read without spectacles.
如果要表示过去做某桩具体事情的“能力”，通常不用could,而用 was /were able to （以及managed to ）.
eg:
He was able to swim halfway before he collapsed.
He was able to translate the article without a dictionary .
I talked a long time ,and in the end i managed to make her believe me .
上述could 与was /were able to 用法上的区别仅是就肯定句而言，在否定句中却无此限制，could 即可表示过去的一般能力，也可表示过去做某桩具体事情的能力。
eg:
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 .
表示“能力”，除用can ,could ,be able to 外，还有其他表示法。
eg:
He has the ability to deal with complicated problems.
You are capable of better work than this .
He is capable of running a mile in four minutes.
2:表示“可能”
表示“可能”，可用may ,might ,can ,could 。 may /might 表示“可能”，往往可以交替使用，并无时间上的差异，只是用might 在口气上比用may 更加不甚肯定，从而比较婉转。may/might 即可表示现在的“可能”
It may /might be true .
He may/might be still waiting at the door .
He may not /might not live near the theatre.
也可表示将来的“可能”
He may /might leave tomorrow .
He may not /might not leave next week .
在疑问卷中表示“可能”，须用can 而不用may .
eg：
Where can he be ?
He may be in the office .
Can they have missed the bus ?
Yes,they may have .
在肯定陈述句中，用can 表示“可能”与用may表示“可能”，往往产生含义上的微妙差别，用can 表示“可能”往往指逻辑上的“可能性”，而用may 表示“可能”则指事实上的“可能性”。
The road can be blocked .=(It is possible for the road to be blocked )
The road may be blocked .=（It is possible that the road is blocked ）
又例如
Mr Reed is in poor health .He can be ill at any time .
Mr Reed looks pale .He may be ill.
表示过去的“可能”，可用may/might 不定式完成时或不定式完成进行时，在这里用may/might ，也只有口气上婉转程度的差异，而没有时间上的区别。
eg:
He may/might have been hurt .
She may/might have missed the train .
用might/could 不定式完成时，有时可以表示本来可能发生，或者本来可能完成却没有完成的动作。
eg:
You might have killed yourself .(你差点儿送了命）
You might have finished the work last week.
I could have reported you .(我本可以告发你）
I could have passed the examination ,but i failed .
表示“可能”，除用may /may not ,can /can't 外，还有一些形容词、副词、以及相应的名词也都能表示“可能”
Frost is possible ,although unlikely at this time of year .(下霜是可能的，尽管在这个时节下霜可能性不大）
She was possibly the greatest writer of her generation .(她可能是她那一代人中最伟大的作家）
There is possibility of his coming /that he'll come .
It is perhaps the best known of her works .
如果要表示很大的可能性，可用 probable ,probably ,likely ,probability,likelihood 等。
eg:
It is probable /likely that he will come .
He is likely to come .
He's probably stuck in a traffic jam.(他很有可能遇上了堵车）
There is little probability of his succeeding /that he will succeed .(他不大可能会成功）
