The examples you are giving are what are often referred to as gerundives of obligation. That's where a gerundive is used to indicate what "has to be" done.
magister audiendus est - the master/teacher/foreman is to-be-listend to, or:
imperium regendum erat - The empire was to-be-ruled.
Another way to translate these sentences is
The master must/has to be listened to.
The empire had to be ruled.
case is often used with this with the translation of "by
E.G. magister servis audiendus est.
The master must/has to be listened to by the slaves. OR The slaves must listen to the master. (The second translation is in more usual English and generally acceptable.)
The tense of the form of sum
dictates what tense to use in English:
Present - must; have/has to
Imperfect - had to
Future - will have to
A perfect participle just indicates that the action of the participle happened before that of the main verb.
There are other uses for the gerund/gerundive, but I've tried to address your concerns with these examples.