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Tom walked into a wall. He can't have been looking where he was going.

a) Can I use mustn't instead of can't here? ("He mustn't have been looking where he was going.")

b) If I use couldn't instead of can't here, will it decrese the probability of my supposition?

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I have answered a similar question of yours here: He mustn't/couldn't have been

To answer succinctly your question, you can use can't have and couldn't have indifferently.

I know that online there are websites that infer can't have = 99% certainty, whereas mustn't have (which is non-standard English)= 95% can't have vs.couldn't have But how do you measure 99%? And why is mustn't fixed at 95%?

b) If I use couldn't instead of can't here, will it decrease the probability of my supposition?

When speaking, listeners will understand how certain you are by your facial expressions and voice intonation. Some will argue that using couldn't tentatively increases uncertainty. I think this is subjective, there is no fixed rule. Use whichever you're most comfortable with, and that includes mustn't have

If however, you are planning to sit an exam in the future, choose between can't have or couldn't have in your writing. Every time.

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