There are four types of conditional clauses in English: Type 0 expresses factual statements. Type 1 expresses possible future conditions using "if + present, will + infinitive". Type 2 expresses unlikely future conditions using "if + past, would + infinitive". Type 3 expresses imagined past conditions using "if + past perfect, would have + past participle". Conditional clauses are used to express possibilities and hypothetical situations depending on certain conditions.