Conditional sentences are composed of an if-clause and a main clause. There are four types of conditional sentences classified according to their implications. Type 0 uses present tense in both clauses to talk about universal truths. Type 1 uses present tense in the if-clause and future tense in the main clause to talk about probable future situations. Type 2 uses past tense in the if-clause and "would" in the main clause to talk about unlikely present or future situations. Type 3 uses past perfect tense in the if-clause and "would have" in the main clause to talk about impossible past situations.