Conditional sentences consist of two clauses: a conditional clause and a main clause. The main clause can only occur if the condition in the conditional clause is met. There are different types of conditional sentences including real/likely conditionals which have a present or future time reference and the condition seems possible to occur, and negative conditions which can be expressed using "unless." Future time clauses use words like "as," "before," and "after" to indicate the condition must be fulfilled at a specific time in the future for the main clause to occur.