THE ACTIVE–PASSIVE ALTERNATIVE
In describing situations which involve two participants, it is usually possible to take one or other participant as Subject and Theme/Topic. This is done in English by means of the active–passive voice alternative:
The President has released the prisoners. (active voice)
The prisoners have been released [by the President]. (passive voice)
In the active construction, the Agent is mapped on to Subject and Theme/Topic, while the Affected is in final position and receives normal, unmarked end-focus:
In the passive construction these correspondences are reversed. The Affected is now promoted to Subject and provides the point of departure, while the Agent is demoted from its privileged position as Subject and is usually omitted. If present, it occupies final position and receives normal end-focus:
We can see that the active–passive alternative allows speakers and writers to exploit the two main positions in the clause, the beginning and the end. In each case, a single clause can be arranged so that important new information is placed in end-position, while already known information is placed at the beginning. What is new and important and what is known is of course estimated by the speaker, and is dependent on the context and the estimated state of knowledge of the hearer at that point in the discourse.
