"The farmers are wounded."
Translation:Los campesinos están heridos.
This is confusing because the English is often ambiguous and Spanish has two different words for the verb "to be."
In this sentence, we're merely connecting the subject "the farmers" with an adjective "injured" via the copular verb "are."
If you use "ser" to translate this sentence, then you are using the passive voice and describing what happened to them. That is, the passive voice says "the farmers are injured (by someone/something)."
If you use "estar" to translate this sentence you are just describing the condition of the farmers. This is not the passive voice.
What makes it doubly confusing is that Spanish also uses "ser" to describe people when speaking of characteristics or intrinsic qualities. If you wanted to describe the farmers as lazy, for example, you'd say "los granjeros son perezosos." If you used "estar," you'd be implying they are acting lazy now, but aren't generally lazy.
With past participles, I believe it's the case that "ser" is only used in passive voice constructions and "estar" is always used to describe states. That's why "he's dead" is "él está muerto" even though death is generally considered a permanent state and not just a temporary condition. If there are exceptions to this, I'd love to hear about them.