La carta fue enviada por María.
English: The letter was sent by María.
The 'voz pasiva' (passive voice) in Spanish is a way to focus on the action or the object of the action, rather than on who does it. In passive sentences, the subject receives the action.
Use the passive voice in Spanish when you want to emphasize the action or the result, and the person or thing doing the action is unknown, unimportant, or obvious.
La carta fue enviada por María.
English: The letter was sent by María.
Las puertas son abiertas a las ocho.
English: The doors are opened at eight.
La película fue vista por muchas personas.
English: The movie was seen by many people.
El examen será corregido mañana.
English: The exam will be corrected tomorrow.