Question

How does an aldehyde sugar differ from a ketone sugar?

Answer 1

An aldehyde group can be easily oxidized into a carbonic acid group.

Explanation:

An aldehyde group `-CHO` can easily be converted into a `-COOH` group, even by mild oxidants.
A ketone group `-C=O` has no `H` attached to the same `C` so it cannot be oxidized, unless by very strong oxidants, which will usually destroy the whole molecule.

This is the basis for Fehling's reagent for glucose in urine samples, an early diagnostic for diabetes.
https://en.wikipedia.org/wiki/Fehling%27s_solution