Mustn’t Have Done and Couldn’t Have Done

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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.

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.)

The question, I suspect, troubles ESL learners more than it does native speakers.

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