How would I identify and translate a purpose clause?

A purpose clause explains why an action is done and so can be considered the motivation behind the verb. It is constructed with 'ut' + a subjunctive verb and is translated in English as 'in order to' or simply 'to'. For example: cives ad portas urbis festinaverunt ut verba nuntii audirent.audirent is the subjunctive verb and the subject is cives (because it is the only nominative, plural noun in the sentence). Therefore following the subject, verb, object pattern, you would translate this sentence as: the citizens hurried to the gates of the city in order to hear the words of the messenger.A negative purpose clause is constructed with 'ne' + a subjunctive verb and is translated as 'so that...not'.For example: Hannibal noctem exspectavit ne ab custodibus videretur.Hannibal waited for night so that he would not be seen by the guards.