The document discusses the four types of conditional sentences in English. Conditional type 0 refers to situations that are always true, using "if/when + simple present, + simple present". Conditional type 1 refers to possible real situations, using "if/will". Conditional type 2 refers to unreal present or future situations, using "if + past simple, + would + infinitive". Conditional type 3 refers to unreal past situations and is used to express criticism or regret, using "if + past perfect, + would + present perfect". Examples are provided for each conditional type.