The document discusses three types of conditional sentences in English. The first type uses "if + present simple" in the if-clause and "will + infinitive" in the main clause to express possibilities. The second type uses "if + past simple" in the if-clause and "would + infinitive" in the main clause to express improbabilities. The third type uses "if + past perfect" in the if-clause and "would + have + past participle" in the main clause to express impossibilities. Examples are provided to illustrate each type of conditional sentence.