Can't have drunk VS Isn't able to have drunk VS Wasn't able to have drunk
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
Solution 1:
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
- 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
- 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".)