Active vs passive voice
In an active clause the participant that carries out the action (the Agent) is also the grammatical Subject of the verb:
Chris (S) posts (V) the letters (O) every day.
In a corresponding passive clause, the grammatical constituent the letters is moved to the front and becomes the subject, while Chris is placed at the end in a by-phrase, which is usually optional:
The letters (S) are posted (V) every day (A) (by Chris) (A).
The active verb posts is replaced by the passive verb are posted The passive verbal structure typically consists of a form of be and a past participle. Verbs which can take the passive are transitive verbs such as post or eat, which take one Object, or those such as such as give, which take two Objects. The latter typically have two passive alternatives.
By using the passive instead of the active voice in all three variants listed above, the Agent can be omitted altogether, leaving other semantic roles to take its place.
By means of such reorganizations of the clausal message, the content of the clause can be made to relate to the rest of the discourse and to the communicative context in which it is produced.
We shall now look at the full range of grammatical units in a hierarchy where the clause is central. We shall then look briefly at the unit above the clause, the ‘complex sentence’, and the units immediately below the clause, the ‘groups’.
