I hear that:
That must not be Toby! - correct
That must be not Toby! - doesn't make sense
Do you agree?
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Sign up to join this communityI hear that:
That must not be Toby! - correct
That must be not Toby! - doesn't make sense
Do you agree?
As FumbleFingers said, "negation is complex". It is especially so when tied up with the evolution of English and with gerunds.
At the time of the King James Bible "Be not" was colloquial: "Be not afraid", for example, where today we'd say "Don't be afraid"; or "If it be not Toby", where today we'd say "If it isn't Toby." Googling "Be not" returns mostly examples of its usage dating from the 16th to the 18th century, together with discussions about English grammar, many of them here on StackExchange!
If, today, it was idiomatic to say "That must be not Toby" then it would also be idiomatic to say "That be not Toby" and "That be Toby". We don't. We say "That is Toby", "That is not Toby" and "That must not be Toby!"
This - on the use of the present subjunctive- might be helpful.