If the moon is made of green cheese then pigs can fly the moon is made of green cheese therefore pigs can fly is a well structured argument?

This is a classical example of a tautology.

The sentence "if [P and Not(P)], then Q" is always true, regardless of the truth values of P and Q.

This is the principle that, from a contradiction, anything (and everything) follows as a logical conclusion.

Essentially, assuming something that can never be true is true ("If the moon is made of green cheese"), you can prove anything.

Thus the statement "If the moon is made of green cheese then pigs can fly" is true.