Passive voice is used when:
- the action is more important than the person who performs it
- when it is not known who did the action
not all verbs can be used in the passive voice
To change a sentence into the passive voice it is necessary to have an agent and an object:
an agent is a person or thing that performs the action
an object needs a verb to make sense in a sentence
it was raining – this sentence does not have an object and therefore it cannot be transformed into the passive voice
I always lose my keys – this sentence has an agent (I) and an object (keys) so it can be changed into the passive voice
my keys are always lost (by me)