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To refer to future ability we should strictly use will be able to in certain instances, while in other cases it is possible to use either can or will be able to. Why? How can one account for that convincingly?

For example,

I'll be able to use a typewriter perfectly after a few more lessons

but

We can/will be able to sit at home tomorrow and watch the match in comfort.

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Strictly speaking, English has no 'future tense'; it constructs future-tensed clauses with present (or non-past) forms, and all present forms and constructions are capable of bearing future-tense understanding. –  StoneyB Oct 30 '13 at 17:08
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And, strictly speaking, there are in fact no circumstances in which "we should strictly use will be able to". All these constructions are strictly optional. –  John Lawler Oct 30 '13 at 17:37
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How about 'By the end of this course you can speak English like a native'? Does it sound natural? –  niab Oct 30 '13 at 18:14
    
In answer to your last comment/question, niab, the sentence "By the end of this course you can speak English like a native" does not sound natural or correct to me. To make it work, I think, you would have to slip into the future by a separate entrance—as, for example: "By the end of this course, you will find that you can speak English like a native." I can't explain why this is the case, but my ear assures me that it's true. –  Sven Yargs Nov 2 '13 at 1:42

4 Answers 4

I think the difference has to to with the particular meaning of the typewriter sentence.

In the latter example, the sports match, you are talking about a hypothetical, something that can, but will not necessarily occur. Can suggests the possible, but is mute about the probable. The phrase will be able also may be used to convey that theoretical possibility.

In the first sentence, you are not talking about a theoretical state of affairs, but a predicted achievement that you fully expect to realize.

I will be able to use a typewriter perfectly after a few more lessons, just you wait and see!

The word perfectly and a few more lessons makes this a very specific predicted outcome, rather than just a possibility.

If you were talking about a more general condition, say, under what circumstances can a person learn to use a typewriter perfectly, you could say

You can learn to use a typewriter perfectly after only a few lessons.

This is again theoretical, not predictive of a particular outcome.

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The first sentence (I'll be able to use a typewriter perfectly after a few more lessons) is clearly about an ability that does not exist now but will exist in the future. This is why it has to be will be able to; can does not work in such a context.

In the second sentence (We can sit at home tomorrow ... ) can is more about a present possibility than a future ability. That is why the present tense can be used.

Here are some more sentences exemplifying the difference:

Our baby will soon be able to walk.

I don't think I'll ever be able to play the piano.

..

The doctor can see you first thing tomorrow morning.

I can provide the sound system for your party on Friday.

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Comparing your answer with what I read in my grammar book and what I know as an almost fluent English speaker is that "can" in the future can't ever be used as an "ability". If I'm wrong, could someone give me a clear example of it?

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The more formal definition of using "can" to mean "will be able to" has really been weakened by years/decades of colloquial use of "can" with meanings centered around simply "able to." The "will be" part, is completely reliant on context. Though it may be definition-correct to use "can" with "will be able to" in mind, writing (IMO [at least in non-poetical cases]) is done with the purpose of explaining or convincing a reader, and both depend on you writing CLEARLY.

Your point should always be clear the first time a reader reads your sentence. If you, the author, have any doubts about the ambiguity of a sentence, the chances your readers will get confused is almost guaranteed.

So, how to account for that convincingly, is to be sure the context of your sentence always makes that distinction CLEAR and INTUITIVE, with zero reliance on the reader thinking something like, "does he mean they could choose to, or they will be physically/legally/emotionally/etc. ABLE to, do __ tomorrow?"

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