How does the subjunctive work in Spanish?

The subjunctive is used in Spanish to present possibility - something that isn't certain. For example, if I'm not sure if it will rain or not, I would say 'Es posible que lluvie' (It is possible that it will rain / There is a chance that it will rain). There are a series of 'trigger' phrases which will need to be followed by the subjunctive, such as 'Es posible que', 'Es probable que', 'Hay una posibilidad que'.It is also used when the subject of a sentence changes - 'Es posible que él vaya a la playa' (It is possible that he will go to the beach).