I have underlined the ones that need to be changed: The police WOULD HAVE never found the corpse if they HADN'T HAD the assistance of an old man. They WOULDN'T HAVE BELIEVED he could help them if he HADN'T PERFORMED some remarkable feats earlier. He said that if he WAS younger, he'D lead them to the body even more quickly than he had: his reactions WOULD be quicker and he wouldn't hesitate a couple of times.
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