The Passive

Many actions (transitive) involve two people or things: one that performs the action and one that is affected by it. The person or thing we want to talk about is usually put in the beginning of the sentence as subject of the verb. So, if we want to talk about the performer, we will make it subject:

    Columbus discovered America, that's why he is so well-known

Now we are talking about Columbus, but if we want to talk about America we may say:

    America was discovered by Columbus and all its civilization changed dramatically

In the first example we are talking about the doer of the action, Columbus, and so we make it subject of the active sentence and America, the thing affected, is the object. In the second example we are talking about America, so we make it subject of the passive sentence and the doer of the action becomes a complement (AGENT) introduced by the preposition BY, or it may disappear altogether.


FORM -

Passive forms consist of an appropriate tense of BE followed by the past participle of the verb. For example, the passive form of the present perfect of  EAT is the present perfect of BE followed by EATEN.

                Active:             Subject  +     Verb     +  Object

                Passive:            Subject  +  Passive V.  + Agent

      Tom broke the window                (we are talking about Tom)
      The window was broken by Tom    (we are talking about the window)


USE-

As we said before, we use a passive sentence when we want to talk about the person or thing affected by the action. In that case, the performer becomes agent, but it is usually not mentioned and it is certainly not so when it is unknown or void of information.

not mentioning the agent-

We omit the agent in the following situations:

- when we don't know it
    he's been murdered
       (you would never say "by someone" or "by them" because it doesn't give any information at all)

- when it is obvious
    he has been arrested
    you will be fired

- when it has already been mentioned
    the firemen arrived at once and the fire was extinguished

- when you don't want to make it known
    the original has been destroyed

- when people in general are the agents
    wine can be bought in a supermarket

NOTE: In this last case there is an alternative active construction with "you", "they" or "one" (more formal) as impersonal subject (void of meaning)
    you can't cut a diamond with a knife
    they have built a new hospital in Salisbury
    one can never find a taxi when one needs it

mentioning the agent-

In a passive sentence, we are talking about the person or thing affected by the action, not about the doer (the agent), so the agent is not mentioned unless it gives important information.
    this picture was painted by Dalí
    the new bridge was inaugurated by the king
    a dog was bitten by a man in Piccadilly

When the agent expresses the instrument or the means, rather than the performer, we usually use WITH instead of BY
    water filled the room the room was filled with water
    a bomb killed him he was killed with a bomb