All sentences are either active or passive.
These two sentences have different meanings:
ACTIVE: John helped the other students.
PASSIVE: John was helped by the other students.
In the active sentence, John did something. In the passive sentence, something happened to John (he did not do anything).
The other students were helped by John.
John is the agent because he did something.
John was helped by the other students.
John is not the agent; the agent is the other students.
We use by before the agent in a passive sentence. But many passive sentences do not tell us about the agent because it is not of interest or not important (Lesson 44).
[AA]:ACTIVE AGENT,[AV]:ACTIVE VERB,[PV]:PASSIVE VERB,[PA]:PASSIVE AGENT
These two sentences have the same basic meaning:
[AA] Maria [AV] helped the other students.
The other students [PV] were helped by [PA] Maria.
Lesson 44 shows you when to use the passive.