Conditional sentences have two clauses: an if-clause and a main clause. There are four types of conditional sentences classified based on probability: zero, first, second, and third conditionals. Each type uses different verb tenses in the if-clause and main clause. Zero conditionals use present tense in both clauses to talk about universal truths. First conditionals use present tense in the if-clause and future tense in the main clause to talk about probable future events. Second conditionals use past tense in the if-clause and "would" in the main clause to talk about unlikely present or future situations. Third conditionals use past perfect tense in the if-clause and "would have" in the main clause to talk about