In economics, oversimplifying slightly, the market price is the equilibrium price. That is, where the demand curve and supply curve cross. This is the price the market will settle on without outside forces acting on it.
A subsidy results in a sale price below the equilibrium price while sellers receive payments above the equilibrium price. An excise tax has the opposite effect.
Different excise taxes on competing products may lead consumers to prefer one over the other, but still results in less consumption of both products than the market would without intervention.
OK, so let's check whether that holds up: Suppose petrol was taxed at 1000 EUR/l and diesel was taxed at 0.01 EUR/l. Do you think that that would lead to less consumption of diesel than in the no-intervention equilibrium with the seller receiving payments below the no-intervention equilibrium price?
A subsidy results in a sale price below the equilibrium price while sellers receive payments above the equilibrium price. An excise tax has the opposite effect.
Different excise taxes on competing products may lead consumers to prefer one over the other, but still results in less consumption of both products than the market would without intervention.