El subjuntivo en general

Hay usos particulares para el subjuntivo y algo casi formuláico en cómo se usa en una frase. Compara estas frases:

• Yo quiero venir a la fiesta. I want to come to the party.
• Yo quiero que él venga a la fiesta. I want him to come to the party. (or more literally, I want that he come to the party.)

La primera frase usa el indicativo. It simply makes a statement about me.

La segunda frase tiene 2 verbos conjugados: quiero y venga. Quiero es indicativo y venga es subjuntivo. In this sentence, I tell what I want someone else to do.

This is a crucial point for knowing whether to use subjunctive or not. Subjunctive is used to express desires, wishes, recommendations, doubts, and emotional reactions. This doesn't mean that every time I say I want something I now use subjunctive. Just like in the examples above, I only use subjunctive if I say I want someone else to do something. I still tell my emotions in the indicative: Estoy contenta. However, if I tell my emotional reaction to what someone else does, I will end up using subjunctive: Estoy contenta que él venga a la fiesta.

Open the following document "Subjunctive Basics" for a more detailed explanation of how to tell when you need the subjunctive.