In Titus Livy's "ab Urbe Condita" 26.1.2:
"Q. Fululo Ap. Claudio, prioris anni consulibus, prorogatum imperium est atque exercitus quos habebant decreti, adiectumque ne a Capua quam obsidebant abscederent priusquam expugnassent." =
Quintus Fulvius and Appius Claudius, consuls of the former year, were continued in command, and the armies which they had before were assigned to them, it being added that they should not withdraw from Capua, which they were besieging, until they had taken it."
At the end the indirect-command clause, "ne...abscederent" = "they should not withdraw". The verb in an indirect-command clause must be subjunctive, by the grammatical rule.
Following on, "priusquam expugnassent." = "until they had taken it." This is not a part of the indirect-command clause, because it is not a command; but, "expugno" is still given in the subjunctive.
Why is "expugno" given in the (pluperfect) subjunctive?
Thanks to TKR for pointing out that this quote from Livy is an indirect command and not negative-purpose clause.