In this change, (1) bear, the object of the active verb shot, becomes the subject of the passive verb was shot; and (2) Richard, the subject of the active verb shot, becomes by Richard, an adverbial phrase, modifying the passive verb was shot. Thus we have the rule:—

The object of the active verb becomes the subject of the passive, and the subject of the active verb becomes in the passive an adverbial phrase modifying the predicate verb.

Active VoicePassive Voice
My cat caught a bird.A bird was caught by my cat.
Austin thanked Charles.Charles was thanked by Austin.
The bullet penetrated a tree.A tree was penetrated by the bullet.
Sargent painted that portrait.That portrait was painted by Sargent.
The fireman had saved the child.The child had been saved by the fireman.

250. Intransitive verbs are ordinarily used in the active voice only.

251. An intransitive verb followed by a preposition is often used in the passive, the object of the preposition becoming the subject of the verb.

Active VoicePassive Voice
Everybody laughed at him.He was laughed at by everybody.
The attorney general has not yet passed upon this bill.This bill has not yet been passed upon.
He has tampered with this lock.This lock has been tampered with.
The cart ran over me.I was run over by the cart.

Other examples are: