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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    "It is able to be possible" is not grammatical in English. You mean: It could be possible. – Lambie Jun 5 at 15:30
  • I was told "It can be posible" was bad. How can "It could be possible" be okay? – Michael Azarenko Jun 5 at 15:35
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    There is no need to say: It can be possible. If it can be possible, it is possible. However, it would be possible is idiomatic, if you believed me. For the past tense re the coffee and can: She could have drunk that much coffee. could is the past of can. – Lambie Jun 5 at 16:28
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    No, it might be possible, I don't know. – Lambie Jun 5 at 17:41
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    It may or might,but not can. – Lambie Jun 5 at 18:10

The short answer is no. The exchanges don't work.

In some contexts you might turn She can't have drunk that much coffee (which is fine) into She isn't/wasn't able to have drunk that much coffee but it's an uncomfortable mouthful.

She wasn't able to.... is far more likely to be used in a context such as She wasn't able to reach the the fruit without using a ladder, referring to someone's physical limitations.

It can be possible that he has forgotten... is not idiomatic. People would simply say It is possible that he has forgotten....

Is able to be possible.... doesn't work at all. We don't generally use able after it. Creatures are able, organisations and governments are able. We even talk about machines and vehicles being able. But it isn't able unless we already know what it refers to. That's to say that you can't use a placeholder it, as in It is able to rain when you mean that it might rain.

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  • 1) Why is "She wasn't able to have drunk that much coffee" okay but "She isn't able to have drunk that much coffee" bad? The difference is in the Tense only. 2) Let's change "possible" for 'true". 'It can be true that he has forgotten" - okay or bad? 3) So, if we talk about a homeless cat, we can't say "It is able to jump high:. If this cat is mine and I love him, speaking of him using "he", I can say "He is able to jump high", but the subject doesn't change - it's still the cat. Sometimes you say "it", sometimes "he". – Michael Azarenko Jun 5 at 14:39
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    1. Both are possible, depending on context. 2. It's clumsy - not idiomatic. We don't talk that way. 3. Once you have made clear that it refers to the cat, yes, it's okay. – Ronald Sole Jun 5 at 15:10
  • 1) As I understand, "She isn't able to have drunk" is correct and possible but it's not used because of being "mouthful" which, as I can see" means "a lot of excessive words". 2) "It can be true" is itself bad or it's bad when it has "that he has forgotten"? – Michael Azarenko Jun 5 at 15:33
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    @MichaelAzarenko No, "she isn't able= present, to have drunk=past: She wasn't able to have drunk. But we actually say: She couldn't have drunk. – Lambie Jun 5 at 22:14
  • Why is "She can't have drunk" okay if "can" - present too? – Michael Azarenko Jun 6 at 1:51

Your first substitution, "She isn't able to have drunk that much coffee." isn't possible because of the tense of "isn't". With "wasn't", it is possible but unlikely.

For your second example, the original sentence, "It can be possible that..." is unlikely itself, because "can" and "be possible" are redundant:
"It could be that he has forgotten" already means "It is possible that he has forgotten..."
("Could" is past tense form of "can".)

That makes the substitution "It is able to be possible that...", basically impossible.

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    First question, yes. Still unlikely, even with "wasn't". The rest of the sentence is speaking about the past, so you can't use the present tense "isn't". Second question, at Google books I find multiple instances of "it is able to be". The ones where "it" has a concrete antecedent seem like reasonable uses; the ones where it is a dummy pronoun, as in your example, seem very unidiomatic. – Jack O'Flaherty Jun 5 at 14:47
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    No, you're mixing up two different answers. As to the tense, "couldn't have drunk" is probably the correct tense, but "can't have drunk" doesn't seem as impossible as "isn't able to have drunk". I'm not sure why. Maybe a new focused question would elicit a better answer. – Jack O'Flaherty Jun 5 at 15:21
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    This isn't right. The utterance She isn't able to have drunk that much coffee might not be particularly likely, but it's syntactically fine, and quite capable of being meaningfully interpreted. – FumbleFingers Reinstate Monica Jun 6 at 15:36
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    @FFRM, I disagree, not on syntactic but on semantic grounds. You'd be saying she is not able, at the present time, to have drunk that much in the past. – Jack O'Flaherty Jun 6 at 15:41
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    @Michael: For your purposes it's probably safer to just say it's not a "correct sentence". I'm simply arguing with Jack as regards whether it might be syntactically invalid and/or incapable of being parsed by a native speaker. But there's really no chance you would ever be likely to use such an utterance "correctly" even if both of us were to agree that it's at least "credible". – FumbleFingers Reinstate Monica Jun 6 at 16:32

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