When is the subjunctive used in Spanish?

In Spanish, the subjunctive mood is used to communicate meanings which are subjective or less certain than the indicative mood. 
The indicative mood is used to express certainty and objectivity; whereas the subjunctive mood expresses doubt, hopes, fears and uncertainty etc. 
Example: 
I doubt that he will go to the beach. 
Dudo que vaya a la playa. 
I hope Julie will return soon. 
Espero que Julia vuelva pronto.