This type of conditional is used to talk about things that did not happen in the past, and how things might have been different if they had happened.
Structure: If + past perfect, would have + past participle.
Example: If I had studied harder, I would have passed the exam.
Note: Sometimes the second and third conditionals can be mixed to talk about hypothetical past situations and their imagined results in the present or future.
Example: If I had a million dollars, I would buy a yacht and sail around the world.