When we want say what people and things do, we use active verb forms such as love, speak, is building, will prepare. When we want say what happens to people and things or what is done to them, we often use passive verb forms such as was loved, is spoken, is being built, well be prepared.
1. John helps Mary.
2. Mary is helped by John.
Both of the above sentences express the same idea but in sentence 1, the form of the verb shows that the person denoted by the subject (i.e. John) does something. Here the verb “helps” is said to be in the Active Voice.
In sentence 2, the form of the verb shows that something is done to the person denoted by the Subject (i.e. Mary). Here the verb “is helped” is said to be in the Passive Voice.
See more examples on the change from the Active Voice to the Passive Voice given below:
Ricky speaks French => French is spoken by Ricky
The carpenter is making the table => The table is being made by the Carpenter
Kamal loves Sita => Sita is loved by Kamal
The guard opened the gate => The gate was opened by the guard.
Few guys were helping the old man => The old man was being helped by few guys.
I will finish the work tomorrow=> The work will be finished tomorrow by me.
Who did this? => By whom was this done?
Why did you write such a letter? => Why was such a letter written by you?
The passive voice is generally preferred when the active voice would involve the use of an indefinite or vague pronoun of noun (somebody, they, people, we, etc,) as subject. That means when we do not know the agent or when it is clear enough who the agent is. In such cases the agent with by is usually avoided. See the following examples.
My car has been stolen. (Somebody has stolen my pen)
English is spoken worldwide. (People speak English worldwide)
I have been invited to the cocktail. (Someone has invited me to the cocktail)
We will do the job quickly. (The job will be done quickly)
Dropping the Agent
Most of the time, the subject of an active voice sentence is dropped in the corresponding passive voice sentence as in the above example. If the doer (also called the “agent”) is required to be given then it can be done with the use of “by” as follows:
ACTIVE: My father built our house in 1975.
PASSIVE: Our house was built in 1975 by my father.
Other prepositions can be used instead of by after the past participles of some stative verbs, including some which are used like adjectives.
We were worried about/by her inability.
Are you frightened of cockroaches?
With is used when we talk about an instrument which is used by an agent to do an action.
He was shot by the robber with a rifle.