Conditional sentences consist of an "if" clause and a main clause. There are four basic patterns of conditional sentences depending on whether the condition is possible, unlikely, or impossible in the present, future, or past. The zero conditional refers to facts that are always true. The first conditional refers to likely or real possibilities in the future. The second conditional refers to unlikely or imaginary possibilities in the present or future. The third conditional refers to impossible or imaginary situations in the past. Mixed conditionals combine elements of past and present.