1. The Agent in Passive Voice
The subject in a passive sentence is being acted upon. The person performing the action is called the agent. The agent is expressed using the dative preposition VON.
example : Ich werde von meinem Freund abgeholt.
I will be picked up by my friend.
If the action of the verb is performed by a thing, rather than a person, that instrument is expressed using the accusative preposition DURCH.
example : Die Bücher werden durch die Schule gekauft.
The books will be bought by the school.

2. A THING AS AGENT (DURCH)
A PERSON AS AGENT (VON)
ACTIVE PRESENT TENSE
Columbus entdeckt Amerika in 1492
PASSIVE PRESENT TENSE
PASSIVE PAST TENSE
A THING AS AGENT (DURCH)
ACTIVE PRESENT TENSE
The library is selling the books.
PASSIVE PRESENT TENSE
PASSIVE PAST TENSE