Of the two sentences below, (1) sounds more natural and acceptable than (2), but I can't explain why. Is (2) as good as (1)? Any semantic difference?

It's only 5 in the morning.
(1) They cannot have arrived yet.
(2) They must not have arrived yet.

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Original Post
Actually, both are natural and acceptable. The two sentences are almost identical in meaning, but not quite. According to some grammarians, (1) shows a slightly greater degree of improbability.

Here's what Betty Azar* says:


- Why didn't Sam eat?

(d) 100%: Sam wasn't hungry.
(e) 99%: Sam couldn't have been hungry. Sam can't have been hungry.
(f) 95%: Sam must not have been hungry.

In (d): The speaker is sure.
In (e): The speaker believes that it is impossible for Sam to have been hungry.
In (f): The speaker is making a logical conclusion...."
"Could not have," "couldn't have," "cannot have," and "can't have" all have the same meaning here.

*Understanding and Using English Grammar, Third Edition, by Betty Azar. Longman, 2002.
Last edited by Rachel, Moderator

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