Complex sentence: Hubbard taught that the thetans brainwashed by Xenu's forces remained on Earth, where the "body thetans", attached to human psyches, contribute to human problems; and that individuals could be freed from these brainwashed thetans and thus attain a type of salvation.
Split into simpler sentences: Hubbard taught that the thetans brainwashed by Xenu's forces remained on Earth. He believed that individuals could be freed from these brainwashed thetans and thus attain a type of salvation.