As you realize, references don’t usually address can’t have vs. couldn’t
have directly.
One that does mention it, however, is Betty Azar, in in Understanding
and Using English Grammar, 3rd ed. (Pearson longman, 2002).
On page 181, she places can’t have and couldn’t
have in
a chart showing their status on a scale of probability. They are equal.
The
title of the chart is “DEGREES OF CERTAINTY: PAST TIME," and the
section of the chart is "PAST TIME: NEGATIVE," (distinguishable from
the section above it called "PAST TIME: AFFIRMATIVE.?
In answer to
the question “Why didn’t Sam eat??
Sam wasn’t hungry.
(The speaker is 100% sure that this is the
reason.)
Sam couldn’t have been hungry.
Sam can’t
have been hungry. (The speaker believes ?is
99% certain –that
it is impossible for Sam to have been hungry.)
Sam must not have been hungry.
(The speaker is making a logical conclusion. We can say he’s about 95% certain.)
Sam may
not have been hungry.
Sam might not have been hungry.
(The speaker is less than 50%
certain,
and is mentioning one possibility.)
So, can’t have and couldn’t
have are
equal in meaning when they express the impossibility of something.
Stylistically, couldn’t
have seems more formal than can’t have.
In writing, could not have would
probably be used, especially in referring to something historical:
George
Washington could not have known Abraham Lincoln ?they
lived at different times.
The original inhabitants could
not have eaten peanuts ?peanuts
were unknown at the time.
When couldn't have is used as a past conditional, can't
have can
not be substituted, in any style:
The team could not/
couldn't [not can't] have won
the game if they had not trained so intensively.
I could
not/ couldn't [not can't] have passed
the course without your help.
My father could not/ couldn't [not can't] have had
the success he enjoyed had my mother not always encouraged him.
See
Marilyn's posting below for an insightful examination of couldn't
have vs. can't have.
Is there a difference in usage between can't have
and couldn't have? They are very similar in their
degree of certainty, as described in Azar. But are they
truly interchangeable, as Anonymous asks? They seem not to
be fully interchangeable, not only in historical writing,
as Rachel points out, but in other contexts as well.*
Can't have seems to be used a great deal in
present contexts where the events being talked about
are recent in time. It expresses a judgment about a
recent action or situation, a context in which the issues
are still fresh and relevant to the present. One use is in
present real conditionals, e.g., in sentence (a):
|
(a)
|
Welcome to my personal page about the eclipse of
the
sun, if you find any of this informative then I can't
have written it right. (If A is true, then B is also
true.)
|
Other uses of can't have express simple belief in
the impossibility of a (recent) past event or situation, as
in (b), (c) and (d):
|
(b)
|
Her favourite film is Kes, and she talks with awe about the
moment when the boy finds the dead bird. She thinks, from the
look on his face, that the child actor can't have known in
advance that the bird was dead.
|
|
(c)
|
If he knew there were blacks in all the other countries, but
he didn't know there were blacks in Brazil, he'd have to have thought
Brazil was somehow an exception
to the rule. But he can't have believed that. No sane person could. Brazil
has the second-largest black
population of any country in the world.
|
|
(d)
|
Please do let me know if you find problems [in the program] as
I'm quite sure I can't have found every
bug.
|
The underlying message of these assertions above — (b),
(c), and (d) — is "It isn't possible (in the present)
that..." It often expresses strong disbelief, or even
refusal to believe something.
Or the speaker may have objective evidence to support
the
idea of impossibility, as in (e), (f) and (g):
|
(e)
|
Tom can't have written this because it is in French and
he doesn't know French.
|
|
(f)
|
I've never written an Improv part. Therefore, I can't have
written bad Improv.
|
|
(g)
|
From what you have said, you can't have known the new
man very long.
|
In each of these cases — (e), (f) and (g) above — the
speaker has a warrant for the assertion being made. And in
every case so far, the topic under consideration is a
recent situation or event rather than a remote past
situation or series of events.
Additionally, can't have is used
to express a
meaning that has nothing to do with possibility. Can't
have is used for present (negative) obligation or
permission. For example, in (h) below:
| (h) |
The Oscar rules state that the song must be recorded
for "use in a film" prior to any other use...i.e., you have to write it for the
film, you can't have written it before (even it was never recorded), and
you have to record it for the film before it can be re-recorded, remixed, etc.
on your own.
|
This use of can't have, which means "You are
not
allowed to have (written...)" is totally different in
meaning from couldn't have, which is not used for
negative permission in the past.
An interesting feature of can't have that
was
revealed in the search is that many instances of its use
are in ES/FL lessons. This fact, together with the ubiquity
of can't have in fiction writing, may say something
about the practical usefulness of the form, suggesting that
its use in everyday speech is probably rather limited. In
fact, the proportion of instances of can't have in
comparison with couldn't have is very small. For
example, can't have known occurs 500 times, whereas couldn't
have known occurs
14,500 times. Can't
have been occurs 15,900 times, but couldn't have
been occurs 229,000 times!
Couldn't have expresses past impossibility of an
idea being true. It has a less complex range of meanings
than can't have.
In addition to its use in full past hypothetical
conditionals, as Rachel has explained, it is found in
incomplete conditionals, utterances in which the if-
clause is merely implied. For example, in (i), (j), and (k)
below:
| (i) |
I couldn't have done it without my family [if I hadn't
had my family]
|
| (j) |
The timing couldn't have been worse [if circumstances
had been different]
|
|
(k)
|
I couldn't have written a better script [if
I'd tried to do it]
|
These ideas have no present tense counterpart with can't
have.
Non-conditional uses of couldn't have abound.
It
occurs to express impossibility in past time narratives or
commentary, as in (l), (m), and (n) below:
| (l) |
[He] couldn't have known that just six hours later, he,
the only male flight attendant on American Airlines Flight 11,
would die suddenly when...
|
| (m) |
As a young woman who had never given birth before, she couldn't
have known what
the experience of
childbirth would be like...
|
|
(n)
|
Athletic Director Edgar Johnson couldn't have
known what he was getting into when he made that fateful decision to cut
the Delaware wrestling program on
June 18
|
With actions that take some ability or skill, it means
didn't have the ability or skill to, as in (o), (p), and
(q) below:
| (o) |
I wish I had written Nowhere Man, by Aleksandar Hemon,
but I couldn't have written it, because no one can write
like Hemon.
|
| (p) |
The imagination was there and so was the humor that was to appear
later in my books, but my teachers didn't seem to notice, except
for one who said, "You couldn't
have written this."
|
|
(q)
|
... So, anyway, I could have possibly written about the incidents,
but I couldn't have written about
them effectively.
|
This use of couldn't have embodies the root meaning
(ability or capability) of the modals can and
could.
To sum up: the two verb phrases share a similar degree of
probability, as seen in Azar. But there are differences in
usage between them:
| 1) |
Can't have tends to be used more in present contexts
or contexts about recent situations or events, while couldn't
have is used more in definitely past contexts.
|
| 2) |
Can't have is found in a lot of fictional writing, perhaps
more than is couldn't have.
|
|
3)
|
Can't have is used to express negative permission, while couldn't
have carries no such meaning.
|
|
4)
|
Can't have is used much less than couldn't have--a
small percentage of usage. This fact may guide teachers in deciding
how much time to spend teaching each of the forms.
|
Marilyn Martin
*The example sentences have been taken from
a Google on- line search.