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If "can" and "be able to" are interchangable sometimes, for example:

She can speak Latin = She is able to speak Latin

Can "be able to" be used instead of "can" in:

She can't have drunk that much coffee = She isn't able to have drunk that much coffee

It can be possible that he has forgotten about us = It is able to be possible that he has forgotten about us ?

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    Both starting examples use can in a wooly way. Then converting to be able to makes them worser, no, worsted. Commented Jun 5, 2020 at 13:30
  • It's some "wool" humor or what? I don't understand Commented Jun 5, 2020 at 14:20
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    If "can" and "be able to" are interchangable sometimes, This is your problem. You have chosen times when they are not interchangeable. She can't have drunk... - this uses "can" to express an probability/possibility. The second one does too but it is not really idiomatic. We would say "It could be possible..." When can is used to express a possibility or request permission, its meaning is not "to be able to".
    – Greybeard
    Commented Jun 5, 2020 at 14:37
  • I was told "It can be possible" was not a good sentence. Using "could" - "Could it be possible" makes everything fine? Commented Jun 5, 2020 at 14:43
  • All of those constructions are syntactically sound. The first example is fine, but the second is unidiomatic and awkward. Commented Jun 5, 2020 at 15:31

1 Answer 1

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The short answer is "no."

In your first example, the infinitive "to speak" is used following the verb phrase "is able." This principle doesn't work with the next example because "to have drunk" is not the infinitive form of "to drink." If you want to use the infinitive form, you would have:

She isn't able to drink that much coffee.

...which is a perfectly fine sentence that would be equivalent to "She can't drink that much coffee." This, of course, is not the same as your original "She can't have drunk that much coffee."

As for your next sentence, "It can be possible that he has forgotten about us," I find that construction awkward and undesirable, but close to the perfectly acceptable question "Can it be possible that he has forgotten about us?"

I can't think of how to make that work with "is able," neither am I able to think of how to make that work with "is able."

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