The document discusses different types of conditional sentences. It explains that conditional sentences contain two clauses: the if-clause which states a condition, and the main clause which states a result. There are four types of conditional sentences - zero, first, second and third conditionals. The zero conditional expresses facts, the first conditional possible future situations, the second conditional unlikely future situations, and the third conditional imaginary past situations. The document provides examples of the structure and uses of each type of conditional sentence.