Sentences in Passive Voice (“Voz pasiva”) are those expressed in terms of a subject receiving (not doing) the action of the verb.
There are 2 options for forming the Passive Voice in Spanish, both being equally valid:
Option 1: Verb “ser” + past participle of the verb (+ por + agent)
Option 2: Particle “se” + 3rd person of the verb
Let’s see each one in detail:
1. Verb “ser” + past participle of verb (+ por + agent)
The first word is the verb “ser” properly conjugated.
The second word is the past participle of the main verb, adapting its ending to match the subject in both gender and number.
La canción es cantada = The song is sung
The verb “ser” has been properly conjugated in the form “es”. Then we have adapted the Past Participle of the verb “cantar” (“cantado”) to “cantada” so that it matches the gender and noun of “la canción”: feminine singular.
In the next example, we will also include the agent of the action at the end of the sentence, introduced by “por”:
Los edificios fueron construidos por la compañía = The buildings were built by the company.
The verb “ser” has been properly conjugated in the form “fueron”. Then we have adapted the Past Participle of the verb “construir” (“construido”) to “construidos”, so that it matches the gender and noun of “los edificios”: masculine plural.
2. Particle “se” + 3rd person of the verb
The first word is always the particle “se”, without any variation.
The second word is our main verb, conjugated for the 3rd person singular or the 3rd person plural in order to match the subject.
La canción se canta = The song is sung.
We have conjugated the verb “cantar” for the third person singular in Present Tense.
Los edificios se construyeron = The buildings were built.
We have conjugated the verb “construir” for the third person plural in Preterite Tense.