In sentences written in passive voice, the subject receives the action expressed in the verb; the subject is acted upon. The agent performing the action may appear in a “by the . . .” phrase or may be omitted

Active- The dog bit the boy

Passive- The boy was bitten by the dog.

Passive: “A pedestrian was smacked by my car.” The subject is the person/thing that got something done to it, and the agent (person/thing that did it) is in a “by” phrase.

More examples:

Active: “Someone stole my wallet.”

Passive: “My wallet was stolen.” (“by someone” is left out)

Active: “Columbus discovered America.”

Passive: “America was discovered by Columbus.”

Does this help?