This document discusses conditional sentences with "if" clauses. It defines conditional sentences as sentences that express an action in the main clause can only occur if a certain condition in the "if" clause is fulfilled. There are three types of conditional sentences: type 1 uses "if + present" and "will + infinitive"; type 2 uses "if + past" and "would + infinitive"; type 3 uses "if + past perfect" and "would + have + past participle". The document provides examples and explanations of each type and their meanings in referring to possible future, unlikely present, or impossible past situations. It concludes with an exercise matching conditional sentences to their types.