when we are using the , but NOT future forms, we use words and expressions such as while, before, after, until/till, as, when, whenever, once, as soon as, as long as, by the time, etc to introduce time clauses.By the time we get there the film will have started
we also use the but NOT future forms after words and expression such as unless, if, suppose/supposing, in case etc. Take an umbrella in case it rains later.