Activity
#3367·Benjamin Davies revised 4 months agoThis might be a difference in dialect. In New Zealand (and I assume other places, like maybe Australia, UK and Ireland) it is common to use ‘must not’ to mean:
a) ‘ Is forbidden to’ (the meaning you are familiar with),
or
b) ‘necessarily cannot’, usually in a deductive way.
Example: “His shoes aren’t here. I guess he must not be home then.”
This is much more natural to me than “His shoes aren’t here. I guess he cannot be home then.”
The other day, I heard an American say ‘must not’ in the sense you mean. So this seems to be more common than I realized.
He didn’t use the contraction, and I suspect Americans would find the contraction unnatural. But they do apparently agree that ‘must not’ does not only mean ‘is forbidden to’ but also ‘necessarily cannot’. So I was definitely wrong about this.