A reader has requested a post on the distinction between “mustn’t have + past participle” and “couldn’t have + past participle.” The reader provides the following examples:
a) Ahmed failed the exam. He mustn’t have studied hard.
b) Ahmed failed the exam. He couldn’t have studied hard.
Prior to contacting me, the reader consulted native English speakers and received varying responses.
• Some native speakers believe that ONLY the first example is correct.
• Others argue that both are acceptable.
• Some suggest that “mustn’t have + pp” implies a conclusion based on evidence.
• Some claim that “mustn’t have” indicates an 80% certainty, while “couldn’t have” implies 100% certainty.
Both a) and b) are considered correct.
The first sentence is more commonly used in British English, while the second is prevalent in American English. Nonetheless, in this context, both constructions with mustn’t and couldn’t can be used interchangeably as speakers speculate on why Ahmed may have failed the exam.
I have come across numerous discussions on the mustn’t/couldn’t distinction in ESL forums. The application of percentages of certainty to grammatical structures is a unique perspective.
Degrees of Certainty
An example from a grammar book illustrates this concept:
When asked why Sam didn’t eat:
“Sam wasn’t hungry.” (The speaker is 100% certain.)
“Sam can’t have been hungry.” (The speaker is 99% sure it’s impossible for Sam to have been hungry.)
Sam must not have been hungry. (The speaker is about 95% certain.)
“Sam might not have been hungry.” (The speaker is less than 50% certain, mentioning a possibility.)
Context determines whether these constructions convey certainty.
Examples where mustn’t have and couldn’t have indicate a conclusion based on evidence:
If the blood was fresh, this murder mustn’t have been too long ago.
From his writing style, he mustn’t be older than 30.
With darkly tinted windows, Snell couldn’t have seen Johnson inside.
She couldn’t have understood the radio broadcast due to the language barrier.
The evidence supporting the conclusion is evident within the sentences.
the freshness of the blood.
the writing style.
the opaque windows.
the language barrier.
Other Scenarios
In contexts lacking internal evidence, assigning percentages of certainty to these constructions is futile.
The following examples may 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. (Possibly he didn’t do it at all)
She couldn’t have tried very hard. (Perhaps she exerted maximum effort but lacked necessary skills.)
ESL learners may find this question more challenging than native speakers do.
