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I don't understand when and why to use can't have been. It seems so strange to me.

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    You can't have been paying attention when your English teacher covered this! :)
    – FumbleFingers
    Commented Dec 12, 2013 at 22:34
  • hahah :) I am from the Czech republic. My English teacher doesn't teach us this kind of things. I think she herself has no idea, because to her we don't have to know more than what is necessary for passing the school-leaving exam. Lame, I know.
    – janie
    Commented Dec 13, 2013 at 14:52

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You're right; can't have been is strange.

For one thing, can is a modal auxiliary verb; all modal auxiliary verbs are irregular as hell.
For another, can't is a negative, and negatives with modals have different syntax.

Of course, a great deal depends on what the predicate of the sentence is.
Can't have been is just a string of auxiliaries, not a predicate.
It could have a predicate noun or a predicate adjective or a predicate complement

  • They can't have been so drunk that they didn't notice their car had been stolen. (pred adj)
  • They can't have been doctors; they were wearing nurse's uniforms. (pred noun)
  • It can't have been playing the wrong note that infuriated him so. (pred comp clause)

In each of these examples, note that the corresponding affirmative sentence is terrible:

  • *They can have been so drunk that they didn't notice their car had been stolen. (pred adj)
  • *They can have been doctors; they weren't wearing nurse's uniforms. (pred noun)
  • *It can have been playing the wrong note that infuriated him so. (pred comp clause)

This is because the epistemic sense of can is a negative polarity item,
and requires a negative trigger. Without a negative, you can't use can epistemically.

This is a fact like the fact that epistemic will ('be scheduled to, be sposta', sometimes called "future tense") is not allowed in hypothetical clauses (*If it will rain tomorrow, the picnic is cancelled).
Very strange syntax and many irregularities with modals.

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    Bill's answer notwithstanding, it seems to me that are many contexts where can/could are effectively interchangeable. For example, I don't see any real distinction between your first example and "They couldn't have been so drunk that they didn't notice". Is there any simple reason why the "corresponding affirmative" doesn't work with your example, but it does with "They could have been so drunk that they didn't notice" (one denies that possibility, the other allows that it might be true).
    – FumbleFingers
    Commented Dec 13, 2013 at 17:41
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    Couldn't is better in all of these cases, because its epistemic sense is very close to that of can, and epistemic could is not restricted to negative contexts like epistemic can. That's why can't have been sounds odd; yes it's grammatical there, but why use a special restricted form when the normal form is couldn't?
    – John Lawler
    Commented Dec 13, 2013 at 17:45
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All right. I've done some research concerning can't have been vs. couldn't have been.

To remind meanings and uses of couldn't have :

  • it is used in past hypothetical conditionals - can't have can not be substituted
  • expresses impossibility in past time
  • embodies the root meaning (ability or capability) of the modals can and could - that they didn't have the ability or skill to.

Can't have and couldn't have share a similar degree of probability. They are equal in meaning when they express the impossibility of something.

The differences:

    • "Can't have been" suggests that it happened more recently. It expresses a judgment about a recent action or situation, a context in which the issues are still fresh and relevant to the present.
      (e.g. Can you check my homework? I'm quite sure I can't have found every bug.)
    • "Couldn't have been" suggests that it was further in the past.
      (e.g. As a young woman who had never given birth before, she couldn't have known what the experience of childbirth would be like.)

  2. Can't have is used to express negative obligation or permission, while couldn't have carries no such meaning. This use of can't have means "You are not allowed to have (written...)."
    (e.g. You have to write it during the class, you can't have written it beforehand.)

Though can't have is used much less than couldn't have.

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You'll find can't have ... ed vs. couldn't have ...ed explained in a detailed way here: http://www.pearsonlongman.com/ae/azar/grammar_ex/message_board/archive/articles/00108.htm

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  • This answer contains no information beyond a link. When Pearson Longman next rearrange their website (and corporations do this all the time), the link will break and your answer will contain no information at all. Please at least give a summary of the page you're linking to.
    – David Richerby
    Commented Dec 20, 2014 at 12:10
  • It's now September 2022 and the link is still working. A very useful link because Pearson is a reliable source.
    – Peter
    Commented Sep 7, 2022 at 5:16

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