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Mustn’t Have Done and Couldn’t Have Done

Mustn’t Have Done and Couldn’t Have Done

A reader has asked for a post on the difference between “mustn’t have + past participle” and “couldn’t have + past participle.” He gives these examples:

a) Ahmed failed the exam. He mustn’t have studied hard.
b) Ahmed failed the exam. He couldn’t have studied hard.

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Before writing to me, the reader queried native English speakers of his acquaintance and received these answers.

• Some native speakers say that ONLY the first example is correct.
• Others say that both are correct.
• Some say that “mustn’t have + pp” indicates a conclusion based on evidence.
• Some say that “mustn’t have” suggests an 80% certainty, whereas “couldn’t have” provides 100% certainty.

Both a) and b) are correct.

The first statement is more likely to be spoken by a speaker of British English and the second by a speaker of US English. Either way, in this context, the speakers are merely speculating as to why Ahmed may have failed the exam. In this context, the constructions with mustn’t and couldn’t are interchangeable.

I have found numerous discussions of the mustn’t/couldn’t dichotomy in ESL forums. I don’t think I’d ever seen percentages of certainty applied to grammatical constructions before.

Degrees of certainty
Here is an illustration from an actual grammar book:

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 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 might not have been hungry.” (The speaker is less than 50% certain, and is mentioning one possibility.)

Rather than assigning percentages of certainty to these constructions, it makes more sense to me to say that sometimes they convey certainty and sometimes they don’t. It all depends on context.

Here are examples in which mustn’t have and couldn’t have do indicate a conclusion based on evidence.

If the blood was still fresh that meant this murder mustn’t have been too long ago.

From the style of his writing he mustn’t be older than 30 years of age.

The car’s windows are darkly tinted, so Snell couldn’t have seen Johnson inside.

She couldn’t have understood the radio broadcast because she does not speak Dutch.

The evidence for the conclusion lies in the sentence itself.

the freshness of the blood.

the writing style.

the windows were too dark to see through.

the listener did not know the language.

Other contexts
Lacking internal evidence, the application of percentages to the “certainty” of the meaning of these two constructions is an exercise in futility.

The following examples can convey ideas other than certainty.

You mustn’t have spent much time in New York. (sarcasm?)

He mustn’t have finished his homework on time. (Maybe he didn’t do it at all)

She couldn’t have tried very hard. (Maybe she tried as hard as she could, but lacked the necessary ability.)

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5 Words You Use All the Time That Were Once Totally Unacceptable

5 Words You Use All the Time That Were Once Totally Unacceptable

Think language rules are set in stone? Think again. This fun and insightful piece from Time Magazine highlights five everyday English words that were once considered unacceptable or even flat-out wrong. It’s a great reminder that language is always evolving—and what’s “incorrect” today might be standard tomorrow.

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Attention language lubbers: the fourth edition of Garner’s Modern English Usage is out this month, and while it’s as full of pedantry as any guide on how to properly wield the English language, the new version is also funnier and more data-driven than any reference you’ve likely come across in school.

By using searchable databases like Google Ngrams, lexicographer Bryan Garner has developed ratios for how often words or phrases, one standard and one upstart, are used in published writing. And he has developed his own five-stage system for how widespread a “mistake” has become, from being something sloppy and offensive-to-the-educated to perfectly unobjectionable.

Garner describes himself as something of a linguistic “epidemiologist,” tracking mistakes that spread within a population, sometimes escalating to pandemic levels and then, perhaps, reaching a point at which human bodies have all adapted to the virus and it becomes the new normal.

Garner provides several charming analogies in his preface for how readers of the book should understand his five-stage system of acceptability, one of which is this:

Stage 1: Audible flatulence
Stage 2: Audible belching
Stage 3: Overloud talking
Stage 4: Elbows on the table
Stage 5: Refined

Take buck naked and butt naked.

While professional editors still tend to stick to the more traditional buck naked—which may have been inspired by “the skin of bare buttocks” being reminiscent of a male deer’s soft hide—butt naked is by far the favorite in popular usage these days, according to Garner’s analysis. He categorizes the use of butt naked as “Stage 4,” meaning everybody’s using it “except a few stalwart holdouts” who might be principled but should also be preparing for defeat.

As Garner says, “There’s almost a universal rule that any two words that resemble each other in some way will be confounded.” Here follow five examples of confounded words that have shifted from being abhorred to standard.

contact (as a verb instead of a noun): Stage 5

We might think of this verb as being as normal as to be or to not be today. But there was a time when using contact as a verb (“Contact me later”) rather than a noun (“Avoid contact with angry bees”) had grammarians snapping their monocles. This is a good example, Garner notes, of a usage that had appeal because it was efficient. “It was a huge bugbear in the 1920s and 1930s,” he says of people using it as a verb. “But people like brevity. They don’t want to say get in touch with, and they don’t want to say call, email, text or leave a note at my door.” Gradually, he says, “it’s lost all the bad odor.”

nauseous instead of nauseated : Stage 4

Have you ever said you felt nauseous? In the traditional sense that would mean you felt like you were capable of causing others to woof cookies, not that you were feeling sick to your own stomach—much along the lines of how poisonous and poisoned work. Garner, who is something of a stalwart holdout himself, views nauseous as a “skunked term,” a label he coined for a word that gives off “a bad order” to someone whether it’s used correctly or incorrectly. His advice: just don’t use it at all.

graduate from instead of to be graduated from: Stage 5

Your great-great-great grandparents, Garner says, might have insisted that they “were graduated from” college, because it was the school that was bestowing a level of achievement on graduates after all. By the 19th century, your grandparents might have “graduated from” college, and if you’re over 30, you probably did too. Young people today, he says, are now dropping the “from” and simply saying they “graduated college,” which he classifies as Stage 3. “I wish I didn’t have uncharitable thoughts every time I hear somebody say I graduated college,” Garner says, “but I do.”

spitting image instead of spit and image: Stage 5

As Garner explains, the original spit and image came “from the notion of God’s using spit and dust to form the clay to make Adam in his image.” In fact, people used to say things like, “Tom is the very spit of his uncle!” But the corruption has become so common that using the original today might not only stop a conversation in its tracks but cause unpleasant face-scrunching. Per Garner, spitting image is now 23 times more commonly used than its precursor.

self-deprecating instead of self-depreciating: Stage 5

The oldest meaning of deprecate is to “pray for deliverance from,” which makes the notion of being “self-deprecating” pretty hilarious, or, as Garner puts it, “a virtual impossibility, except perhaps for those suffering from extreme neuroses.” And yet over time, this word has taken on the meaning of that slightly longer word that looks so much like it: depreciate means to belittle, to lessen the value of. These days someone might even try to correct you if, in an attempt to note someone was being (overly) humble, you said they were self-depreciating.

Did this just happen because the shorter word is easier to say? Partly, Garner says. Then again that makes little sense when trying to account for why people use the less-standard preventative instead of preventive or irregardless instead of regardless, he notes. Some linguists would argue that there’s no point fighting against slips like that—that language is forever unfixed and deviations should simply be observed and even appreciated—or that it’s silliness to tell people to follow rules that are as arbitrary as the meaning assigned to a certain jumble of letters. But Garner is not one of them.

“There are a lot of people who mistakenly think intelligibility is the standard. ‘Oh, you knew what I was saying.’ Well, that’s not the standard. That’s a really bottom-of-the-barrel standard,” he says. “People who are concerned with English usage usually want to have their words taken seriously, either as writers or as speakers. And if you don’t use the language very well, then it hard to have people take your ideas seriously. That’s just the reality.”

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Now Open: Scholarships for Attending the 2025 Annual Meeting!

Now Open: Scholarships for Attending the 2025 Annual Meeting!

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Shannon Senior Historian & Assistant Director @ Robert H. Smith International Center for Jefferson Studies

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