Question

what happens when H2 + O2 combine

Answer 1

it forms H2O (water) it also forms H2O2.....

Answer 2

H2 + O2 combines with each other at high temperature to form H2O2. But this is a compound which dissociates at high ir at room temperature to form H2O and O2.
2H2O2 ---> 2H2O + O2