Conditional sentences are used to express situations that depend on certain conditions. They typically consist of two clauses: the "if" clause, which states the condition, and the main clause, which describes the result. For example, in the sentence "If it rains, we will stay indoors," the outcome of staying indoors depends on the condition of rain. There are different types of conditional sentences, including zero, first, second, and third conditionals. The zero conditional expresses general truths, while the first conditional deals with real future possibilities. The second conditional refers to hypothetical situations, and the third conditional discusses past events that did not happen.