In the passive sentence, the object of an active verb becomes the subject of the passive verb.Only transitive verbs are used in the passive.
Intransitive verbs such as happen, sleep, come and seem cannot be used in the passive.

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)

Agent
In a passive clause, we usually use a phrase beginning with by if we want to mention the agent - the person or thing that does the action, or that causes what happens.If you want to change an active sentence which has two objects into its passive forms, there are two ways:
1. Make its indirect object into the subject of the passive sentence.
2. Make its direct object into the subject of the passive sentence.Pattern: be + past participle

Example:
- The boy are listening to a story.The story was being told by grandfather
- Snow white eats a poisonous apple
A poisonous apple is eaten by snow white
- Snow white is eating a poisonous apple
A poisonous apple is being eaten by snow white
- Snow white has eaten a poisonous apple
A poisonous apple has been eaten by snow white
- Snow white ate a poisonous apple
A poisonous apple was eaten by snow white
- Snow white was eating a poisonous apple
A poisonous apple was being eaten by snow white
- Snow white had eaten a poisonous apple
A poisonous apple had been eaten by snow white
- Snow white will eat a poisonous apple
A poisonous apple will be eaten by snow white
- Snow white is going to eat a poisonous apple
A poisonous apple is going to be eaten by snow white
- Snow white will have eaten a poisonous apple
A poisonous apple will have been eaten by snow white.