Hard carbon is the most commercially viable anode for sodium-ion batteries, with low operating potential, high reversible capacity, abundant raw materials and low manufacturing cost. However, its intrinsic structural disorder, characterized by randomly stacked turbostratic graphene layers, nanopores and amorphous regions, has sparked long-standing debates on fundamental sodium storage mechanisms, significantly impeding rational electrode design and industrial translation. This review overviews sodium storage processes in hard carbon, analyzes synergistic contributions of intercalation, pore filling and surface adsorption, and establishes microstructural-electrochemical performance correlations. It focuses on four mainstream modification strategies: Precursor engineering, pore structure regulation, heteroatom doping and interface engineering, elucidating their enhancement mechanisms and discussing inherent limitations and trade-offs. Finally, it outlines key challenges and future directions, providing critical theoretical and technical guidance for next-generation hard carbon anodes and practical sodium-ion battery deployment.