Page 178 - Basic English Usage

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When we say what people or things do, we use active verbs. (For
example built, speak, is repairing, will change.)
When we say what happens to people or things — what is done to them
—- we use passive verbs. (For example was built, is spoken, is being
repaired, will be changed.)
The object of an active verb corresponds to the subject of a passive
verb.

ACTIVE:

PASSIVE: is done (by A).

Active or passive?
We often prefer to begin a sentence with something that is already
known, and to put the ‘news’ at the end. Compare:

Your little boy broke my kitchen That window was broken by your
window this morning. little boy.

In the first sentence, the hearer does not know about the broken
window. So the speaker starts with the little boy, and puts the ‘news’ —
the window — at the end. In the second sentence, the hearer knows
about the window, but does not know who broke it. By using a passive
structure, we Can again put the ‘news’ at the end.
Another example:
John's writing a play.’ ‘I didn't know that.’
‘This play was written by Marlowe.’ ‘Was it? | didn’t know that.’