Bonjour,
Réponse :
1. The killer poisoned the woman with cyanide.
The woman was poisoned ( by the killer )with cyanide
2. He stabbed Felicia and her sister with a knife.
Felicia and her sister were stabbed ( by him ) with a knife.
3. White owners didn't treat slaves kindly.
Slaves weren't treated kindly by white owners.
4. Harriet Tubman helped many slaves during the Civil war.
Many slaves were helped by Harriet Tubman during the Civil War.
Explications :
PASSIF:
BE ( au temps de la voix active ) + participe passé
Ici, toutes les phrases sont au prétérit :
was / were +participe passé
Le COD de la voix active devient sujet de la voix passive.
Le sujet de la voix active devient complément d'agent introduit par "by"
( le complément d'agent peut ne pas être exprimé lorsqu'il est sous-entenndu. )