Sentences may be expressed in the active or the passive voice. The passive is alive and well in the Spanish language. There is no attempt to suppress the passive in non-academic writing as one finds in English.
Examples:
Active voice:
Ella escribió el libro. (She wrote the book.)
Passive voice:
El libro fue escrito por ella. (The book was written by her.)
The passive voice is formed by using the verb ser with the past
participle of the principal verb. The past participle must
agree in number and gender with the subject of the verb. To
indicate an agent who performs the action, the preposition
por is used. To summarize, the construction of the passive
voice is as follows:
Subject + Ser + Past Participle + Por + Agent
Example:
Roberto + fue + atropellado + por + el coche.–> Roberto fue atropellado por el coche.
Some other examples:
Although in Spanish the passive voice is used frequently, it has some notable substitutes.
The impersonal "they"
As in English, when there is no expressed agent of the action,
an impersonal use of the subject pronoun "they" and the verb may
replace a true passive voice construction. Of course, syntactically
this is an active voice construction.
Reflexive substitute for the passive
When there is no expressed agent of the action, frequently a
reflexive substitute for the passive will be used.