1. When the subject is not important or unknown,
2. When the object is more important than the subject,When rewriting active sentences in passive voice, note the following:
- the object of the active sentence becomes the subject of the passive sentence
- the finite form of the verb is changed (to be + past participle)
- the subject of the active sentence becomes the object of the passive sentence (or is dropped)
"New houses will be built in the city."
To practise go to this link!