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If you purchase this then you will be able to do that.

How can I write the quoted statement using can?

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Why? I can't immediately think of reason to use can instead of could. There might be some obscure case when can is correct (or at least accepted). That's the thing about English - there is always an exception – mgb Sep 12 '11 at 17:17
Could sounds like some past events. – Chankey Pathak Sep 12 '11 at 17:18
Could describes a possibility, "If you buy a computer tomorrow you could use the internet". – mgb Sep 12 '11 at 17:26
Hmm okay. Thank you :) – Chankey Pathak Sep 12 '11 at 17:30
"When you buy this computer, you can play the newest games." is similar to what @Martin points out - a possibility - but a strong one; it's almost a certainty you will buy the computer. It seems to me it would be unlikely, for example, for this to occur on packaging, and "you'll be able to" would be more common, but I think I would use "can" just as easily in speech (American English). – aedia λ Sep 12 '11 at 17:52
up vote 4 down vote accepted

Will and can are both modal verbs, and in most varieties of English* there is only one slot for modals, so they cannot both occur at the same time. There is some discussion on that issue here: Can I say "We don't must", any alternative using a modal verb if I can't?

As others have pointed out, the future is not precisely necessary here. English embedded clauses tend to take present tense, so the following is probably close enough for your meaning.

If you purchase this then you can do that.

There is some relevant discussion on that matter here:

Why do I instinctively want to use the present tense with a conditional?

*= There are dialects that allow some "double modals," like might could, but as far as I know, this doesn't include will. In any case, it isn't standard.

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Uhm, no? "You can do that" and "you will be able to do that" are two totally different statements. – RiMMER Sep 12 '11 at 18:39
I'd like very much for "will can" to be the future version of the colloquial idiom "used to could". I can't do that thing now, but I will can! – Dan Ray Sep 12 '11 at 20:14

Will be able to is the future tense of can, therefore the sentence you presented is as close as you can get to express what you're trying to express.

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If you purchase this then you can do that.

However, I would leave the sentence the way it is, as it reads more formally. "Can" has a very colloquial feel to it. It depends on what you're using it for, of course.

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