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.

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.

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