英语：单词Can和be able to的区别

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