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Consider the following:

He will be able to do it.

He can do it.

They mean the same thing, right?

Can "can" replace "will be able to" in any sentence? What is the difference, if anything? Why not just use the shorter version?

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Without a more precise context, it's not really possible to uneqivocably say your two sentences mean exactly the same. But since you suggest that they do, let's assume you have in mind a context where that is in fact the case - so it's just a matter of style which you use. Note that most people probably talk too much anyway, so they're certainly not likely to base their choice on which one is shorter. I suggest you use the short one, which will leave you more time to peruse all the answers you'll get explaining how even in your context the two examples don't mean the same. –  FumbleFingers May 21 '11 at 22:20
    
Have you thought about accepting some answer, or updating your question, if the current answers don't provide what you're looking for? –  RiMMER May 29 '11 at 1:45
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4 Answers

up vote 13 down vote accepted

No, they're not the same thing.

Will be able to obviously talks about a future event, while can talks about the present.

I can swim, so we should go to the pool.

Means I can swim already, I learned it before and I'm capable of doing it now.

I will be able to swim sometime in the future, as long as I take swimming lessons.

Means I cannot swim right now, but I'm hoping to learn in the future.

Probably you meant to ask the difference between can and am able to. If so, there is a difference, yes. Let me explain:

I am able to swim, but I can't, because my skin is damaged.

This means that I am capable of swimming, but I am prohibited to do so, because of an infection. So to put it to an explanation:

To be able to talks about an ability to do something, but it does not mean you are allowed to do such.

Can talks about something you are allowed to do, or about your ability to do it. This is therefore ambiguous.

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Except "I won't be able to swim tomorrow, because I have a skin infection" is perfectly good English. When you put it into the future any meaning of "can" turns into "will be able to". –  Peter Shor Nov 14 '12 at 11:59
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I think the question here is not between can and be able to but rather between can and will be able to. As per my knowledge, can says that he is capable of doing it, and it does not say whether he is going to or not, whereas will be able to says that he is going to do it (in the future). You may want to use will be able to if you want to express your willingness to do it.

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One context where you would have to say “able to” is something like this:

You have to be able to swim every stroke in order to join the swim team.

In this context, “can” isn’t usable. The only other alternative would be:

You have to be capable of swimming every stroke in order to join the swim team.

Stroke = butterfly, back stroke, breast stroke, and freestyle.

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This answer can be improved by adding citations: reliable facts and references which show that the answer is correct. –  MετάEd Sep 19 '12 at 17:39
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You cannot tell the differences now, but you will be able to tell them after receiving our answers.

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protected by Will Hunting Nov 14 '12 at 12:20

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