Recently, I learnt in a previous question that the periphrastic past passive voice in Spanish takes ''Ser''. Example: ''Las 3 bicletas fueron robadas.'' Why then did I see in a textbook "Francia y Espana estuvieron unidas''? Separately, why is it ''unidAs'' and not ''unidos''? Thanks
The periphrastic passive is indeed constructed with ser. What you have in this example is a different periphrastic construction, with estar plus a participle.
The NGLE says “it is formed with participles which express the resulting state of an action or a process” and also that “often a correspondence can be established between the construction with estar (Las puertas están cerradas) and the compound tense of the passive with ser (Las puertas han sido cerradas)”.
Estuvieron unidas means the two countries became united (as in, they were joined in an alliance) and they remained so. They were in a state of being united as a result of a previous action.
If you used the passive voice, fueron unidas, that would mean an entirely different thing: that the two countries were united by someone or something. That is, the sentence would be about the fact that a process or event united the two countries.
There is no corresponding distinction in English, but English uses different strategies to show this (when needed). For example, in English you would say that two countries went to war united, or remained united during the war, or so (for estuvieron unidas). For the passive, English would just say they were united by someone or something.
Some more examples:
- Tres bicicletas fueron robadas. This is just the periphrastic passive. Someone stole three bicycles; the bicycles were stolen.
- Tres bicicletas *estuvieron robadas. This is wrong because it doesn't make sense. Stealing something is an action, "being robbed" is not a state.
- Las puertas fueron abiertas. The doors were opened (by someone).
- Las puertas estuvieron abiertas. The doors were open (during a given time period); they were opened and remained so up until a certain moment.
- Una persona estaba caída en el suelo. A person was on the ground; a person lay fallen on the ground (as a result of having fallen before).
- Una persona *fue caída en el suelo. This is ungrammatical because caer is intransitive, so it cannot have a passive voice; you cannot say someone "was fallen by" something.
- Una persona fue arrojada al suelo. A person was thrown to the ground. Regular passive voice.
- Una persona *estuvo arrojada al suelo. Nonsensical; arrojar is an active verb, things can be thrown but not remain thrown.