This document discusses the second conditional, which refers to an unreal or hypothetical situation. It notes that in the second conditional, the past simple tense is used in the if-clause (e.g. "if I were") while could, might, or would are used along with the base form of the verb in the main clause. Examples are provided to illustrate the structure, such as "If he didn't live by the river, he couldn't go fishing" and "If I won the lottery, I would buy a big house."