sammy16 – They could save Buckbeak because they had already done so. That is, they could go back in time because back in time they had already gone back in time. They never did so with Harry’s parents or Sirius. It is a bit circular and paradoxical but that is how it’s explained in POA (Harry can do the patronus to save his past self because when he was his past self he saw his future self doing it). It’s a narrative device, that’s all….
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