Read the following sentences:

• John killed the spider.
• The spider was killed by John.

You will probably have noticed that both sentences express the same idea. But in sentence 1, the form of the verb shows that the person denoted by the subject (John) does something.The verb killed is therefore 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 (the spider). The verb was killed is therefore said to be in the passive voice.

Compare:

• John loves Alice. (Active)
• Alice is loved by John. (Passive)
• The masons are building the house. (Active)
• The house is being built by the masons. (Passive)
• The peon opened the door. (Active)
• The door was opened by the peon. (Passive)

Notes
When the verb is changed from the active voice to the passive voice, the object of the verb in the active voice becomes the subject of the verb in the passive voice.

Thus in sentence 1, Alice who is the object of the active verb loves becomes the subject of the passive verb is loved.

Only transitive verbs can be used in the active voice because an intransitive verb has no object.

Related posts:

1. Transitive and intransitive verbs
2. Subject and Predicate
3. What are Relative Pronouns and What Do They Do?