subjunctive verbs - how are they used
I see each verb has a subjunctive form. Where are they used and why?
2 Answers
One important thing to remember is that the subjunctive is a mood rather than a tense. 
Now to your question, when you are using verbs of influence, desire, command etc if there's a change of subject - eg, if you want John to do something then you use the subjunctive mood to express the second verb, preceded by que.
Quiero que vaya Juan.
I want John to go.
But if there is no change of subject then you use the infinitive.
Quiero ir.
I want to go.
I hope that helps
Verbs in the subjunctive are used in a few ways, at a high level they're used to express doubt / uncertainty and can also be used to express personal feelings and demands.
A few examples on doubt and uncertainty:
Estoy buscando a un empleado que hable español. I'm looking for an employee who speaks Spanish. (I don't know if one exists or not)
Quiero un pastel que lleve chocolate, cacahuetes, y espárrago. I want a pastry filled with chocolate, peanuts, and asparagus. (I don't know if one exists or not...hopefully not.)
Espero que tengas un buen viaje. I hope you have a good trip. (I don't know if you will...but I hope so.)
A few examples of personal feelings or opinions
Es horrible que haya hambre en el mundo. It's horrible that there's hunger in the world.
Es una vergüenza que exista gente que se maltrata los animales. It's a shame that there are people who mistreat animals.
A few examples of demands
Pedimos que el gobierno pare la guerra. We ask the government to stop the war.
Exigimos que las empresas respeten los leyes We demand companies to respect the laws.
