This document discusses conditional sentences that contain an "if" clause. It defines two types of conditional sentences: true and untrue. True conditional sentences use the present or future tense to express real possibilities, while untrue conditionals use the past or past perfect tense to express unlikely or impossible scenarios. The document further categorizes conditional sentences into three types based on the tenses used in the if and main clauses, and provides examples of each type. It also explains how to determine the factual situation based on the tense combination in the conditional sentence.