单词Can和词组be able to都表示“能够”做某事，在某些情况下可以互换，但在其它情况下则不能。下面简单说一下Can 和be able to的用法及区别。
1, Can 和be able to用于一般现在时，两者可以换
- I can’t swim.
- I am unable to swim （be unable able to的反义词组）
上面两个句子中，be able to相对更正式些。
2, Can 和be able to用于一般过去时，形式变为could或was able to / were able to
2.1 两者表示一般情况下“能够”做某事时可以互换
- I could swim (or I couldn’t swim ) when I was 7 years old
- I was able to swim (or I was unable to swim ) when I was 7 years old
2.2 当用来表示过去特定事件中，“能够”做某事时，要用was able to / were able to。比如，一次划船的经历中，我掉下了水，但我靠自己返回了船上，可以说
- I was able to swim back to the boat.
此句表示过去特定的这个事件中，我可以游回到船上。这种情况下，我们不能说：
- I could swim back to the boat.
但是，如果表示过去特定情况下“不能”做某事时呢？比如我掉下了船，但自己是靠别人救上来的，因为我不会游泳。这时就couldn’t或wasn’t able to/was unable to都可以用。
- She wasn’t able to swim back to the boat
- She was unable to swim back to the boat.
3, 如果在将来时、现在完成时和过去完成时呢？这种情况下只用词组be able to，而不用单词can. 同时，unable to比not able to要更正式。
3.1, 将来时
- You’ll be able to speak really good English by the end of the year.
3.2, 现在完成时
- We’ve been unable to complete the project within the specified budget.
- How long has your little boy been able to read?
3.3 过去完成时
- He was given a detention because he hadn’t been able tofinish his homework.
4,尽管can和can’t 一般不用于将来，但可以用can来表示将来可能的行为。
- I can join you at the meeting, but I can’t stay very long.
5, Can 和be able to的使用方法汇总表
肯定句 |
否定句（或否定含义） | 疑问句 | |
| 现在时 | I can work
I am able towork | I can’t work
I am not able to work I am unable towork | Can I work?
Am I (un)able to work? |
| 过去时 | I could work
I was able towork | I couldn’t work
I was not able to work I was unable to work | Could I work?
Was I (un)able to work? |
| 将来时 | I will / shall be able towork | I won’t / shan’t be able to work
I will / shall be unable towork | Will / shall I be (un)able towork? |
| 现在完成时 | I have been able to work | I haven’t been able to work
I have been unable to work | Have I been (un)able towork? |
| 过去完成时 | I had been able to work | I hadn’t been able to work
I had been unable to work |
Had I been (un)able towork? |
