Question

Hydrogen is a burning gas. Similarly oxygen helps burning but why is water (H2O) not burning ?

Answer 1

The end product of this combustion is, in fact, water, or dihydrogen oxide (H2O), to use the chemical term. In other words, water is hydrogen that is already oxidized, so it can't further oxidize (burn). Water doesn't burn for the same reason ashes don't burn— because it is already burnt.