The second or the third conditional?

(The situation: The Town Council is considering to demolish the old city park.)

Which of the following questions is "more correct"?

What would happened if this park was demolished?

What would have happened if this park had been demolished?

I am not sure.

Answer

Neither sentence is acceptable.

The first is grammatically unacceptable.

• The modal auxiliary would must take an infinitive as its complement:
  

  
  

  What would happen ...

  
  

  
  


• 
  

  And in formal use the verb in the condition clause should take the irrealis form, since would in the then clause is irrealis:

  
  

  
  

  ... if the park were demolished.

  
  

  
  
  

  (However, was is acceptable in non-formal use.)

  
  

The second is semantically unacceptable in the circumstances you describe. The past perfect in the conditional clause and the irrealis modal past in the consequence clause mark this as a question about the past, not the future: you are asking about the past consequences of a past demolition, which would only be acceptable if you were indulging in historical speculation.

You have two choices:

a. What will happen if the park is demolished? or
b. What would happen if the park were demolished?

These both ask about the future consquences of a future demolition. The only difference between them is the speaker's attitude toward the demolition: in a. she thinks it quite possible that the park will be demolished and wants to know the likely consequences, whereas in b. she thinks it unlikely that that the park will be demolished but is curious about the hypothetical consequences.