MgF2 + H2
Fluorine now has a subscript of 2 due to the fact that Magnesium has a charge of two; swap and drop rules applied means that it switches over to Fluorine.
Hydrogen has a charge of two as it's one of the diatomic elements, meaning it has a charge of two when alone. The other diatomic elements are Fluorine, Oxygen, Nitrogen, and the Halogens.
Hope this helps!