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I found a good sentence:

She can't have drunk that much coffee

It seems to be correct. Why isn't the next thing correct then if "can" and "be able" can mean the same thing?

She isn't able to have drunk that much coffee

I was told that it would be correct if we wrote the sentence with "wasn't":

She wasn't able to have drunk that much coffee

Why so?

Can't have drunk - correct

Wasn't able to have drunk - correct

Isn't able to have drunk - bad

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    Could you please give the context in which you think She isn't able to have drunk that much coffee might be a valid sentence. :)
    – Greybeard
    Commented Jun 6, 2020 at 15:46
  • Should there be some context to define whether it's correct? Can't I simply equal it to "She can't have drunk that much coffee"? Aren't these sentences both correct and don't they mean the same thing?
    – Michael Azarenko
    Commented Jun 6, 2020 at 16:17
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    Yes, there should be context. Honestly, it is impossible to overestimate how important context is in English. Context really does make a difference.
    – Greybeard
    Commented Jun 6, 2020 at 16:30
  • Okay, let's have context. I come back home. I have a sister and a brother. We all like coffee. Unfortunatelly, I can't find anything though we've bought 5kg coffee recently. I ask if my brother knows something. He says that our siter has drunk everything. I say "She can't have drunk that much coffee" and my brother says "Yeah, she is not able to have drunk that much coffee but it's true!".
    – Michael Azarenko
    Commented Jun 6, 2020 at 16:35
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    Thanks, but your brother's comment makes no sense - if she is not able to do that, then it cannot be true. Your brother should reply: "Yeah, I did not believe that she was able to have drunk that much coffee but it's true!"
    – Greybeard
    Commented Jun 6, 2020 at 16:45

1 Answer 1

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I replied in a comment to your question in this post. It is able to be possible/ She is able to have drunk. To expand upon this:

She isn't able describes her current [in]ability.

to have drunk that much coffee describes a past action.

In this context, a current ability cannot describe a past action.

Your choices are

  1. Expressing a fact/belief/opinion

A: “Look! Twenty empty coffee cups! This is a mystery! She isn't able to (can’t) drink [i.e. currently] that much coffee.” Or

A: “The coffee machine is empty and Maria has disappeared. She wasn't able to (couldn’t) have drunk that much coffee, [i.e. at some time in the past] therefore some other people must have been here.” Or

  1. Expressing a probability/possibility

A: “That litre carafe of coffee is empty! I know that the cat likes coffee, and I left her alone for five minutes, but she wouldn't have been able to drink (couldn't have drunk) that much coffee.” [i.e. in the time allowed and because of its size] (The cat is assumed to be alive) Or

A: “I don’t think that it was coffee that killed her. Maria drank a lot of coffee every day but she wouldn't have been able to (couldn't) have drunk that much coffee.”

(You have also introduced "if" clauses - these have their own guidance: you should research "if conditionals".)

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  • 1) I understand "She isn't able to (can’t) drink" and I understand "She wasn't able to drink" but I don't understand "She wasn't able to have drunk". As far as I can see "to have drunk" is past of "she wasn't". If current [in]ability in present can't be combined with "have done", how can [in]ability in past be combined with "have done" if "have done" is past of "wasn't able"? It's symmetrical.2) I understand "She wouldn't have been able to drink" but I don't clearly understand "She wouldn't have been able to have drunk". Can I equal it to "I wouldn't have believed she had drunk"?
    – Michael Azarenko
    Commented Jun 6, 2020 at 17:51

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