In the active voice, somebody is doing something.
Ella escribió las cartas. → She wrote the letters.
Ellos pusieron la mesa. → They set the table.
In the passive voice, something is done (by someone).
Las cartas fueron escritas por ella. → The letters were written by her.
La mesa fue puesta por ellos. → The table was set by them.
When you want to clarify who did the action when using passive voice, there is a simple formula you can follow: recipient of the action (subject) + some form of the verb ser + past participle + por + doer of the action.
Los partidos fueron jugados por los atletas. → The games were played by the athletes.
Los artículos serán escritos por los periodistas. → The articles will be written by the journalists.
Los autos serán vendidos por el vendedor. → The cars will be sold by the salesman.
This is what you hear when you at the end of so many commercials when they say “Se habla español.” It essentially means “Spanish is spoken.” This form of passive voice is very simple to use. Simply use the reflexive pronoun “se” and conjugate the verb for whatever the subject is.
Se habla español → Spanish is spoken.
Se hablan inglés y alemán. → English and german is spoken.
Se hace así. → It is done this way.
Aquí se venderán coches. → Cars will be sold here.
Se compró la computadora. → The computer was bought.
Third person passive
This is essentially the same as in English when people refer to an unspecified “they”.
Inventaron un nuevo tipo de auto. → They invented a new type of car.
Aprobaron una ley. → They passed a law.
Hablan español aquí. → They speak Spanish here.