In the passive sentence above the agent is still present, indicated by the preposition door. However, it is also possible to omit the agent without making the sentence ungrammatical:

This is a very significant characteristic of the passive voice, because it provides an opportunity to talk about actions where it is either (a) unnecessary to report who the agent is, or it is (b) obvious who the agent is or (c) the agent is unknown. This shows that the passive voice can be used when a different perspective is needed.

(a) Agent is unnecessary

The following example could be an instruction by a worried parent to a child planning to go out in the evening.

It is not necessary to spell out who should collect the child: it is more important to address the fact that s/he should not go out (and come home) alone. The focus of the sentence is therefore on the action itself rather than anyone carrying it out.

(b) Agent is obvious

The following example could be a report in a newspaper.

In this example it is not necessary to say who arrested the terrorist as this job is usually carried out by police officers. The focus of the sentence is therefore on the terrorist being arrested rather than anyone carrying it out.

(c) Agent is unknown

The following example could be the opening of a conversation between friends.

Here, the speaker does not know who has stolen the bicycle and is therefore unable to mention the agent. The focus of the sentence is therefore entirely on the theft.

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