The passive periphrastic is a Latin construct to express a sense of necessity in an action. It's called passive because it is expressed in a passive tense, so it's usually better to change it back to its active form when translating. It always has the same structure: subject + gerundive + the verb esse (declinated appropriately) + dative of agent. When translating the subject becomes the object, the verb will include some form of "must","need to" or "have to", the dative becomes the subject. Example: Carthago delenda est (nobis) = Carthage must be destroyed (by us).