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? No, they're not the same
thing. Will be able to obviously talks about a future event,
while can talks about the present. Means I can swim already, I
learned it before and I'm capable of doing it now. 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: 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. You cannot tell the differences now, but
you will be able to tell t