Complex sentence: However, when production of the film began, Yuasa saw how poorly Tokuma Shoten was handling things and knew that Gamera's reputation could never recover from the film, so he had Takahashi rewrite the ending into one where Gamera was killed at the end.
Split into simpler sentences: However, when production of the film began, Yuasa saw how poorly Tokuma Shoten was handling things and knew that Gamera's reputation could never recover from the film. So he had Takahashi rewrite the ending into one where Gamera was killed at the end.