How could I have thought there wouldn’t be another tricky question about perfect modal verbs? Modals and their many meanings stump us teachers from time to time. It’s easy enough to explain the basics, but part of the challenge is understanding where there’s overlap and where there isn’t.
I addressed one question regarding perfect modals in reported speech some years back. More recently I considered the difference between could do and could have done. Now a new question has been posted by a YT viewer.
QUESTION: Do can’t have and couldn’t have mean the same thing?
ANSWER: First of all, can’t have sounds a little strange to my ears. That’s probably because I don’t use it often myself. I think that’s a clue about usage. Perhaps the form can’t have exists and is taught in some grammar books, but it’s probably a lot less common than couldn’t have.
Second, we’re only talking about the negative forms. In the affirmative, I only use and teach could have for a past possibility: You could have told me sooner! Can have doesn’t work at all in that context.
Now let’s get into the overlap in meaning. Azar and Hagen list couldn’t have and can’t have among the modals for past certainty (186). The verbs are given equal strength on their chart: 99% certainty. So if something can’t have been true it also couldn’t have been true. Either way we’re saying that it was nearly impossible.
However, in a conditional sentence we only use couldn’t have for past possibilities:
- I couldn’t have made it on time if I hadn’t taken a taxi.
- He couldn’t have won without his coach’s training. (= if the coach hadn’t trained him so well).
Over a decade ago, Rachel Spack Koch opened a discussion on the Azar Grammar Exchange, and Marilyn Martin contributed some insights on additional uses of can’t have. Marilyn considered recent past events being discussed at the present time. For example, if I ask my son to sweep the floor and he finishes in under a minute, I can remark with great suspicion, “You can’t have swept the entire floor so quickly!” This is a past impossibility. I’m expressing my present belief about a situation in the recent past.
Marilyn confirmed my hunch about can’t have being less frequently used over all, but she proposed one more use of can’t have: negative permission, as in rules regarding eligibility. I found an example in an article on Perth Now about being jobless vs. being officially unemployed. Among the criteria for a survey, the writer listed: “You can’t have been employed – either for pay, or for no pay in a family farm or business – for an hour or more in the survey week.” I hadn’t thought of this use. So while we may have little opportunity to use can’t have this way, the meaning nevertheless exists, so our ears should be ready to understand it correctly if encountered. It’s interesting to note that the example also refers to the recent past. Could this factor into the choice of verb forms?
Have you ever taught the meaning and use of can’t have? I’d love to hear more examples.
Azar, Betty S. and Hagen, Stacy A. (2009). Understanding and using English grammar. White Plains, NY: Pearson Education.
Shiilson-Josling, G. (2012, March 24). Jobless outnumber officially unemployed. Retrieved from http://www.perthnow.com.au/business/jobless-outnumber-officially-unemployed/story-e6frg2s3-1226308962413
Martin, Marilyn. (2003, February 4). Azar grammar exchange. Retrieved from http://www.pearsonlongman.com/ae/azar/grammar_ex/message_board/archive/articles/00108.htm