The beggar was laughed at by the boy.
Given sentence is in Past indefinite (Past simple) tense and it is in the active voice. To change it into Passive voice Object (the boy) will become subject and subject (The beggar) will be object. We also use helping verb of past simple tense was with V3 form of the main verb. Keep it in mind that the preposition at must be retained with the verb.
Several items in the room were destroyed by the burglar. Even the carpet was torn.
Above it contains two sentence one is in Past Simple sentence (Active Voice) and Second sentence is in Present Perfect tense (Passive Voice). We change First sentence according to voice rule and second sentence also be change according to same rule but tense of both sentence become same.