The basicity of phosphorus oxyacids is determined by the number of P-OH bonds (not total H atoms). H3PO2 (hypophosphorous): 1 P-OH + 2 P-H → monobasic. Strong reducing agent (P-H bonds are reactive). H3PO3 (phosphorous): 2 P-OH + 1 P-H → dibasic. Also a reducing agent. H3PO4 (phosphoric): 3 P-OH + 0 P-H → tribasic. Not a reducing agent (no P-H). H4P2O7 (pyrophosphoric): 4 P-OH → tetrabasic. Two PO4 tetrahedra share one O (P-O-P bridge). HPO3 (metaphosphoric): 1 P-OH → monobasic. Rule: P-H bonds do not ionise in water; only P-OH bonds release $H^+$.