1) players have elected him captain of the team.
ANS.The captain of the team had been elected by the players.
.2) The traders have hoarded the grains.
ANS.The grains had been hoarded by the traders.
3) has anyone answered your question??
ANS.Has your questions answered by anyone?
In passive voice the action is acted upon on the subject and the receiver is the object. In simplier words the most important thing in the sentence becomes the subject.