The passive voice is used to focus on the action being done rather than the person or thing doing it. It's formed by using a form of "to be" (is, are, am, was, were) followed by the past participle of the main verb. It's used when the focus should be on the action or when the doer is unknown or unimportant. For example, "The book was written by the author." To form the passive voice, follow this structure: (subject) + (form of 'to be') + (past participle) + (by + agent - if necessary).