7. The Passive is Used:

A.When it is not necessary to mention the doer of the action as it is obvious who he is/was/will be:

The streets are swept every day. You will be X-rayed.

B. When we don’t know, or don’t know exactly, or have forgotten who did the action:

The minister was murdered. I’ve been told that...

C. When there is an ‘empty’ subject of an active verb such as you, one, they, people or someone:

Active Passive

You/One should check the details. The details should be checked.

They’re increasing the rents. The rents are being increased.

People use this footpath every day. This footpath is used every day.

Someone took my purse. My purse was taken.

The passive form is more usual here, though in colloquial speech we can use the ‘empty’ subject and the active verb.

D. When we are more interested in the action than the person who does it:

The house next door has been bought (by a Mr Jones).

If, however, we know Mr Jones, we would use the active:

Your father’s friend, Mr Jones, has bought the house next door.

E. The passive may be used to avoid an awkward or ungrammatical sentence. This is usually done by avoiding a change of subject. So the sentence When he arrived home a detective arrested him would be better expressed When he arrived home he was arrested (by a detective).

F. The passive is sometimes preferred for psychological reasons. The speaker may know who performed the action but wish to avoid giving the name. Tom, who suspects Bill of opening his letters, may say tactfully:

This letter has been opened! instead of You’ve opened this letter!

8. Passive Sentences with or without by:

A. In a passive sentence, when we want to say who or what did the action, we use by:

We were stopped by the police.

The paper was all blown away by the wind.

In a passive sentence we mention the agent, the person or thing doing the action, only if it is important. We do not mention the agent when:

1. the agent does not add any new information:

The money was stolen.

The men were arrested last night.

We do not need to say that the money was stolen by a thief or that the men were arrested by the police.

2. the agent is not important:

Oil has been discovered in Bavaria.

Who discovered the oil is less important than the fact that it is there.

3. it is difficult to say who the agent is:

This kind of jacket is considered very smart.

A number of attempts have been made to find the Loch Ness monster.

B. We can give other details about the action. For example, when we want to say when or where something happens, we can use in, to or at:

The telephone was invented in 1876.

The visitors will be driven to the airport.

The concerts are held at the university.

C. After some past participles which are used like adjectives (to describe a state of mind, not an action), other prepositions are used instead of by to introduce the agent. Compare:

She was frightened by a mouse that ran into the room.

I’ve always been terribly frightened of spiders.

D. With is used when we talk about an instrument which helps the agent to do an action.

He was shot with a revolver.