This can be difficult to answer in words outside a classroom setting, but usually it is used to describe a wish, a desire, a hypothesis, a probability, etc Quiero que comas toda la comida. Es posible que vengan tarde hoy. It usually expresses something that you want to take place, or may take place, but actually hasn't happened yet. Hope this helps.

The "rule" I have found to work the best is fulfillment. If something is not or has not been fulfilled then the verb usually takes the subjunctive. None of the "rules" work perfectly and the subjunctive can be very tricky. There is a thin grammar book called "The Spanish Subjunctive; Up Close" by Practice Makes Perfect. It is well worth it and makes the Subjunctive (all forms) much seem much easier and more straightforward.

http://users.ipfw.edu/jehle/courses/PERSUAD1.HTM Here is a link that may help you. It is something that you will have to study in great detail. If you google ''Spanish subjunctive'''you will see many links like the one that I have sent you, try and read as many as possible. ¡Que tengas suerte!

As someone who is also learning Spanish, I can tell you that you need to see it many times to understand the subjunctive. BUT.... essentially it is almost always used to indicate something that is uncertain. "I want you to know this" = "Quiero que sepas esto". I am not sure if you will know, so I cant say "sabes". I say the subjunctive: sepas, which indicates the uncertainty. Espero que aprendas el subjuntivo. buena suerte. -sven