Time clauses are used to explain when a future action will take place. They are formed with two clauses: a time expression clause using words like when, before, after, etc. followed by a subject and present tense verb, followed by a second clause with a subject, will, and verb to describe the future event. The order of the two clauses can be reversed without changing the meaning. Common time expressions used in time clauses include when, as soon as, before, after, by the time, once, and until.