Please tell me the passive form (if there is) of the following sentence:
Leave at once.
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Imperatives can be changed to passive by using "let"
"Eat the cake" - "Let the cake be eaten"
However, in "Leave at once", the main issue is not with the phrase being an imperative but with the verb "to leave" being, in this case, intransitive (i.e. has having no direct object).
"John left", similarly cannot be turned into passive voice, however, "John leaves the pen behind" can be changed to passive: "The pen is left behind (by John)"
Similarly "(John) leave the pen behind" can be transformed to "(John) Let the pen be left behind"
'Leave at once' is in the imperative mood. I don't think there is a way of putting it in the passive, at least I can't think of one.
However even if it was written indicatively, 'He left at once', it is difficult to see how a passive could be formed, because there isn't even an implied direct object.
But if 'leave', were used transitively, in the indicative e.g. 'He left the building', you could form a passive - 'the building was left by him'.
I am willing to bow to any grammarian who can offer better than this.