Electrochemical reduction of biomass-derived feedstocks holds great promise to produce value-added chemicals or fuels driven by renewable electricity. However, mechanistic understanding of the aldehyde reduction toward valuable products at the molecular level within the interfacial regions is still lacking. Herein, through tailoring the local environments, including H/D composition and local H₃O⁺ and H₂O content, we studied the furfural reduction on Pb electrodes in acid conditions and elucidated the detailed pathways toward three key products: furfuryl alcohol (FA), 2-methylfuran (MF), and hydrofuroin. By combining isotopic labeling and electrokinetic studies, we revealed the source of protons (H₂O and H₃O⁺) plays a critical but different role in the hydrogenation and hydrogenolysis pathways toward FA and MF, respectively. In particular, the product-selective kinetic isotopic effect of H/D and the surface property-dependent hydrogenation/deuteration pathway strongly impacted the generation of FA but not MF. This is because FA and MF are produced from Langmuir-Hinshelwood and Eley-Rideal pathways, respectively. Through modifying the double layer by cations with large radii, we further correlated the product selectivity (FA and MF) qualitatively and quantitively with interfacial environments (local H₃O⁺ and H₂O content, interfacial electric field, and differential capacitances). Experimental and computational investigations further suggested competitive pathways toward hydrofuroin and FA: Hydrofuroin is favorably produced through the self-coupling of ketyl radicals in the electrolyte, which are formed from the outer-sphere single-electron transfers, while FA is generated from hydrogenation of the adsorbed furfural/ketyl radical on the electrode surface.