Evaluate the long-run and short-run sustainability of export subsidy? The question is base on export subsidy.

The notion of an export subsidy is typically considered to be an unfair business practice in most cases.   

Let's say that companies in country A have created a car which runs on air.   Everyone knows that this kind of car is going to sell all over the globe.

The government in country A sees the advantage in not having companies in other countries  developing  their own version of this kind of car.   There are many reasons for this perspective which I will get in below. 

In order for country A to create a near-monopoly in the manufacture of this type of car, they tell domestic companies that they will give them $5,000 for every "air car" that they build for export.  

Companies in countries B, C and D see that the import "air car" is selling  in their countries for thousands of dollars less than their own manufacturing costs  would be.    Building this type of car in countries B, C, and D would not only be unprofitable - it would be a financial disaster

So while countries B, C and D postpone or cancel plans to build an "air car", companies in Country A are becoming experts in the technology and the names of their cars, their brands, are becoming respected and household names all over the world. 

This implementation is very advantageous to companies in country A.   They are selling cars by the millions, reaping enormous profits - but it is being done in way that is unnatural - actual business forces have been removed artificially.

It is likely that Company A can't keep up with providing these subsidies - for every car exported they lose $5,000 and seemingly get nothing in return - only the companies benefit.

There is one factor that may convince Company A to see the subsidies as helpful to its overall economic conditions.  While the air-cars are being sold exclusively throughout the world, 200,000 laborers are employed in the building of these cars.   These employees pay federal taxes on their income and, while employed, these people, and their families, rely very little on expensive government programs - they don't need financial help.

But, when other countries realize what country A is doing to be so successful in this industry, there will be a backlash against their country.  Other countries can start a trade war, create huge tariffs on imports, or create subsidies of their own for products that have expertise building or growing.

Ultimately the unfairness of the subsidy program will be damaging to country A.