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 the 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:

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.).

Form of Passive
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 (or is dropped)

como se llega al pasive voice?