This document discusses different types of conditional sentences in English: - Zero conditional refers to situations that are always true in the present or future, with an if-clause in the simple present and a main clause also in the simple present. - First conditional refers to possible present or future situations, with an if-clause in the simple present and a main clause using "will + infinitive". - Second conditional refers to unlikely present or future situations, with an if-clause in the simple past and a main clause using "would + verb". - Third conditional refers to impossible past situations, with an if-clause in past perfect and a main clause using "would have + past participle".