Mustn’t Have Done and Couldn’t Have Done


By Maeve Maddox

A reader has requested for a submit on the distinction between “mustn’t have + past participle” and “couldn’t have + past participle.” He provides these examples:

a) Ahmed failed the examination. He mustn’t have studied arduous.
b) Ahmed failed the examination. He couldn’t have studied arduous.

Earlier than writing to me, the reader queried native English audio system of his acquaintance and obtained these solutions.

• Some native audio system say that ONLY the primary instance is right.
• Others say that each are right.
• Some say that “mustn’t have + pp” signifies a conclusion based mostly on proof.
• Some say that “mustn’t have” suggests an 80% certainty, whereas “couldn’t have” supplies 100% certainty.

Each a) and b) are right.

The primary assertion is extra more likely to be spoken by a speaker of British English and the second by a speaker of US English. Both manner, on this context, the audio system are merely speculating as to why Ahmed might have failed the examination. On this context, the constructions with mustn’t and couldn’t are interchangeable.

I’ve discovered quite a few discussions of the mustn’t/couldn’t dichotomy in ESL boards. I don’t assume I’d ever seen percentages of certainty utilized to grammatical constructions earlier than.

Levels of certainty
Right here is an illustration from an precise grammar guide:

In reply to the query “Why didn’t Sam eat?”:

“Sam wasn’t hungry.” (The speaker is 100% certain that that is the explanation.)

“Sam can’t have been hungry.” (The speaker believes – is 99% sure –that it’s unimaginable for Sam to have been hungry.)

Sam should not have been hungry. (The speaker is making a logical conclusion. We are able to say he’s about 95% sure.)

“Sam might not have been hungry.” (The speaker is lower than 50% sure, and is mentioning one chance.)

Fairly than assigning percentages of certainty to those constructions, it makes extra sense to me to say that typically they convey certainty and typically they don’t. All of it depends upon context.

Listed here are examples by which mustn’t have and couldn’t have do point out a conclusion based mostly on proof.

If the blood was nonetheless recent that meant this homicide mustn’t have been too way back.

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

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

She couldn’t have understood the radio broadcast as a result of she doesn’t converse Dutch.

The proof for the conclusion lies within the sentence itself.

the freshness of the blood.

the writing fashion.

the home windows had been too darkish to see by means of.

the listener didn’t know the language.

Different contexts
Missing inside proof, the appliance of percentages to the “certainty” of the which means of those two constructions is an train in futility.

The next examples can convey concepts apart from certainty.

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

He mustn’t have completed his homework on time. (Perhaps he didn’t do it in any respect)

She couldn’t have tried very arduous. (Perhaps she tried as arduous as she might, however lacked the mandatory skill.)

The query, I believe, troubles ESL learners greater than it does native audio system.

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