We use active voice when we say what people and things do. We use passive voice when we say what happens to people and things.
Study the following sentences.
Active voice: My father built this house in 1986.
Passive voice: This house was built in 1986 by my father.
Active voice: This book will change your life.
Passive voice: Your life will be changed by this book.
Active voice: The boy killed the snake.
Passive voice: The snake was killed by the boy.
Passive voice is used when we want to talk about an action, but are not interested in saying who or what does or did it. Passive verb forms, for example, are common in academic and scientific writing.
The pyramids were built around 400 AD.
The object of an active verb corresponds to the subject of the passive verb.
Active voice: Jane wrote a story. (Subject – Jane, Object – a story)
Passive voice: A story was written by Jane. (Subject – a story, object – Jane)
In most cases the subject of the active verb is not expressed in the corresponding passive sentence. If it has to be expressed, we usually introduce it with ‘by’.
This house was built in 1980. (Here the subject of the active verb is not expressed.)
This house was built in 1980 by Richard Nixon. (Here we use the preposition ‘by’ to introduce the subject of the active verb – Richard Nixon.)