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)