This is going to be a long answer, but bear with me.
Your example is not exactly the "voz pasiva". This is rather the "voz pasiva refleja".
This is very complex to explain, because we have many things involved in the examples you found, because the translations tend to be pretty much the same in English, and because this is horribly confusing for native speakers as well, but I'll try to give many examples. Do not be afraid to ask any questions you need to ask until it's clear for you, Bob.
The passive voice in Spanish is exclusively formed by the verb "
ser" + "
past participle", and it is not that there is no subject, but rather that the subject is implicit in the sentence. Also if you can tell
by whom the action is being performed, then it is the passive voice.
- Un acuerdo unilateral
ha sido establecido (
por el presidente).
A unilateral agreement
has been established (
by the president).
- La casa
fue restaurada (
por los dueños).
The house
was restored (
by the owners).
- Los niños
son vacunados (
por enfermeras) en el hospital.
The children
are vaccinated (
by nurses) in the hospital.
- Las actividades
serán asignadas (
por nuestros supervisores).
Activities
will be assigned (
by our supervisors).
Now, we also have a particular construction, which is called "voz pasiva refleja", which may be confused with impersonal sentences:
First of all, the sentences where there is no subject, the
verb is conjugated in third person singular and there is a "
se" involved, are impersonal sentences:
-
Se pide a los clientes no entrar con mascotas.
Clients are asked not to come in with pets.
- Aquí
se respeta a todo el mundo.
Here everyone is respected.
-
Se te
ve cansada.
One can perceive you look tired.
- No
se debe dejar a los niños solos.
Children must not be left alone.
In these examples, there we don't know who asks the clients not to bring in dogs, who respects everyone, who sees the person is tired, or who must not leave children alone. All these are impersonal sentences, although grammatically, the translations have led me to have one in English.
Finally, at last, the "voz pasiva refleja". These sentences also have "
se", but there is a "
sujeto paciente", someone or something we can identify, always a third person, that can be singular or plural, and the
verb agrees with the person. And, although the construction introducing an active subject is normally not found in common texts, in the "voz pasiva refleja", you might also know
by whom the action has been performed.
In your example, if we write the sentence in the active voice, we would have a
subject,
a verb and a
Direct Object:
-
El presidente ha establecido un acuerdo unilateral.
When we change it to the "voz pasiva refleja", we find that the
Direct Object from the active voice has become the "
sujeto paciente" in the "voz pasiva refleja":
-
Se ha establecido un acuerdo unilateral (
por el presidente).
So, both in the "voz pasiva" and the "voz pasiva refleja", there is a
subject, but it's not explicit.
The main differences are:
· The "voz pasiva" is constructed only with "ser" + "past participle", while the "voz pasiva refleja" is built with any verb.
· The "voz pasiva" may be built with any person, but the verb in the "voz pasiva refleja can only take a third person, either singular or plural.
· A sentence with "voz pasiva" never starts with "se".
More examples of "voz pasiva refleja":
- En México
se comen muchos insectos.
In Mexico many insects are eaten.
- No
se dicen malas palabras en la mesa.
One must not say bad words at the table.
-
Se venden helados.
Ice-cream is sold here.
- Aquí
se habla español.
Spanish is spoken here.
-
Se oye un pájaro que canta.
A bird can be heard singing.