[krinchan]

What is all this? As a native English speaker, I view mustn't as a contraction of must not; other examples include cannot and can't, should not and shouldn't, and do not and don't.

So the usage of mustn't should be exactly the usage of must not because that's literally what it is.

"You must not spend time on US conspiracy forums."

"Must" is an imperative or a command. "Should" is a recommendation.

[91bananas]

Long time after the fact, but this was my understanding of contractions also, and was my intended usage of "mustn't". English is a crazy language, especially when we all seem to have been shipped with different compilers.

[throwanem]

I mean you're not wrong in the narrowly denotative sense, but there's a distinction to be made between the way words are defined and the way they're conventionally used. I'm describing my observation of the latter. (So are you! But you observe differently.)