The document discusses the third conditional, which refers to hypothetical situations in the past that are contrary to reality. The third conditional uses the past perfect simple in the if-clause (e.g. "if I had had") and the past conditional (would have + past participle) in the main clause (e.g. "I would have bought"). It provides examples comparing the second conditional, which refers to present and future situations, to the third conditional, which refers to past situations that did not actually occur.