Subject + 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 form of the verb to be is changed (to be + past participle)
the subject of the active sentence becomes the object of the passive sentence (or is dropped)
Passive voice is used when the focus is on the action. It is not important or not known who or what is performing the action.
Example: My bike was stolen.
In this example, the focus is on the fact that my bike was stolen. I do not know who did it.