A reducing sugar has a free anomeric carbon $\frac{hemiacetal}{hemiketal OH}$ that can open to give a free -CHO or undergo tautomerism to an aldehyde.

Reducing: glucose, fructose (enediol tautomerism), maltose (free C1 on second glucose), lactose (free C1 on glucose unit). These all give positive Tollens' (silver mirror) and Fehling's (red Cu2O) tests.

Non-reducing: SUCROSE — the glycosidic bond involves BOTH anomeric carbons (C1 of glucose and C2 of fructose), so neither can open to a free carbonyl. No free hemiacetal/hemiketal = no reducing ability.

Hydrolysis of sucrose gives "invert sugar" (equimolar glucose + fructose) — the optical rotation changes from dextro (+66.5 degrees) to levo (-20 degrees) because fructose is strongly levorotatory. This inversion gives the mixture its name.