15.7.1 Basic rules for the flexion of the participe passé

when using in compound tenses
verbs that form their compound tenses with être are to be inflected
Il est allé. He has gone.
  Elle est allée. She has gone.
  Ils sont allés. They have gone.
  Elles sont allées. They have gone.
in the passive voice it is always inflected, even if the compound tense is formed with avoir.
Il est trompé. He is betrayed.
  Elle est trompée. She is betrayed.
  Ils sont trompés. They are betrayed.
  Elles sont trompées. They are betrayed.
  Il a été trompé. He has been betrayed.
  Elle a été trompée. She has been betrayed.
  Ils ont été trompés. They had been betrayed.
  Elles ont été trompées. They had been betrayed.
verbs forming their compound tenses with avoir, are not to be inflected *
Il a acheté une voiture. He has bought a car.
  Elle a acheté une voiture. She has bought a car.
  Ils ont acheté une voiture. They have bought a car.
  Elles ont acheté une voiture. They have bought a car.

Only in case there is a direct object before the participe passé, then it is also to be inflected. But some more about this later.

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