What is the difference between "Can’t have happened" and "couldn’t have happened"?
Regarding the sentences below, how should I use both expressions differently?
"This can’t have happened if ..."
"This couldn’t have happened if ..."
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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.
http://www.pearsonlongman.com/ae/azar/grammar_ex/m...
can't means can not. you wouldn't say this can not have happened because the tenses of can and have conflict. you say it can not happen (can't happen) or could not have happened (couldn't have happened) or would not (wouldn't) have happened