Mustn’t Have Done and Couldn’t Have Done

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FromDaily Writing Tips:

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.

Link to the rest at from Daily Writing Tips