rationale of するんじゃない as informal negative imperative I know verb- can be used in colloquial speech as a negative imperative (e.g. ) in lieu of verb- (), but why exactly does it function that way? That is, what is the rationale/etymology behind v- being a negative imperative?

Obviously it's not grammatical imperative, but the construction _functions_ as order when used by somebody's betters (senior, superior etc.) to strongly admonish them. If I can ignore context, "You don't want to do --!" could be a way of translation.

* V()- (more pompously V()-): affirmative command
* V()- (V()-, V()-): negative command



Incidentally, the dictionary form is also employed as command, with colloquial but very overbearing tone.

* V(): affirmative
* V()-: negative