① 如果引导的条件可以实现，主将从现;

② 如果引导的条件和事实相反，则用虚拟语气;

eg：If winter comes can spring be far behind.

As long as it doesn't rain, we can play.

You can go out on condition that you wear an overcoat.

Modern cars are far tougher to steal, as their engine management computer won't allow them to start unless they receive a unique ID code beamed out by the ignition (点火) key