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“Sanatana” means “that which always exists” — the eternal. Hindu Dharma holds that the foundational principles of dharma — truth (satya), compassion (daya), kindness (karuna), good deeds (sukarma), and devotion to God (Ishvara-bhakti) — have always existed and always will. They were not invented at some moment in history. For this reason, Hindu Dharma is called Sanatana Dharma — the Eternal Way.
This sets the stage for everything that follows. Because the dharma itself is eternal, no single person could ever have “started” it — which leads naturally to the next question, about who its founder is.