Talonflame's "Jet Shoot" attack says, "during your opponent's next turn, any damage done to this Pokemon by attacks is increased by 40 (after applying Weakness and Resistance)." But what if on the next turn the opponent attacks with a Fighting Pokemon that normally only does 10 damage? Since Talonflame has -20 Resistance to Fighting, does the damage reduce to zero and then +40 for the "Jet Shoot"?

No, since the damage was reduced to zero there is no damage done for Jet Shoot to increase. The attack ends up doing zero damage, period.

Relevant cards