The document discusses the zero conditional, which refers to situations that are always true. It provides examples of zero conditional sentences using the structure of "if" clause followed by a main clause. Specifically, it states that if you heat water to 100 degrees it will boil, if water gets below freezing it will turn to ice, and if you add two and two the result will be four. It then provides exercises to match if clauses with main clauses to form additional zero conditional sentences.