Just remember this: the agent of a sentence is the one doing the action of the verb. Consider the following sentences:
(1) The car struck the fence.
(2) The fence was struck by the car.
In both (1) and (2) it is the car that is doing striking, even though in (2) it is NOT the subject. It is the subject that does the action of the active sentence, and the noun of the prepositional phrase in the passive (if the sentence has a prepositional phrase at all) that is the agent.
Saussure pointed to that language is mistakenly thought of as a matching of a thing to a name. To him the link is between a concept (signified) and a sound pattern (signifier). The signified is its meaning and the signifier is the “container”. The two together makes the linguistic sign.
The linguistic sign has two characteristics. One is that the link between signified and signifier is arbitrary. There is no natural link or reason that the concept should connected to its “container”. Secondly, the signifier is linear temporally and physically. It is a thing in its own right.
Furthermore, the value of a sign is summarised thus:
A language is a system in which all elements fit together, and in which the value of any one element depends on the simultaneous coexistence of all the others.
And so, “in the language itself, there are only differences“.
Reference:Course in General Linguistics, Saussure.
One of the reasons (there are many reason but this is just one) why we would like to change an active sentence into a passive one is because we would like to bring the object of the sentence into focus. Consider these sentences:
My brother was hit by a car.
A car hit my brother.
The focus on my brother is far more important than that of the car. So it would seem logical to put my brother in the subject position of the sentence, as in 1. The subject position should be seen as being reserved for more important information, or be the focus of the sentence. So if the sentence becomes a passive structure let it be so if it is appropriate. But when the choice of active or passive is an equally valid one, choose the active. The active is usually clearer and more efficient.