The document summarizes the three main types of conditional sentences in English:
1) First conditional - used to talk about possible future events. The if-clause uses the present simple and the main clause uses the future simple.
2) Second conditional (hypothetical) - used to talk about unlikely future events or present situations that are impossible. The if-clause uses the past simple and the main clause uses would + infinitive.
3) Third conditional - used to talk about imagined past situations that did not happen. The if-clause uses the past perfect and the main clause uses would have + past participle.