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, the focus is on the fact that the 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, example: A mistake was made. In this case, I focus on the fact that a mistake was made, but I do not blame anyone (e.g. You have made a mistake.).
Subject + 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.