Transitive verbs can be used in two different ways, called voices: active and passive.
Active: A vicious Rottweiler dog attacked them.
Passive: They were attacked by a vicious Rottweiler dog.
In the passive voice it is as if the object of the sentence gets a voice of its own and can describe an event from its own point of view.
This applies even if the original subject is inanimate:
Active: A big wave hit the side of the paddle wheel.
Passive: The side of the paddle wheel was hit by a big wave.
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