Passive Voice

Use of Passive

passive voice is used when the focus is on the action. it is not important or not known, however, who or what is performing the action.
Example: My bike was stolen.
In the example above, the focus is on fact that my bike was stolen. I do not know, however, who did it. Sometimes a statement in passive is more polite than active voice, as the following example shows: A mistake was made.
In this case, I focus on the fact that a mistake was made, but I do not blama anyone (e.g. You have made a mistake).

Form of Passive

S + finite form of to be + past participle (3rd column of irregular verbs)
Example: A letter was written.
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)

Sumber: 

htpps://www.ego4u.com/en/cram-up/grammar/passive