When should I use the present subjunctive in Spanish?

The subjunctive is a mood, rather than a tense. A verb is triggered into the subjunctive when expressing a wish, hope or desire. For example, "I want you to play the piano" becomes "quiero que -toques- el piano". Also, you use the subjunctive mood when you wish to express doubt or uncertainty. For example, "I doubt that it will happen" is translated as "dudo que -suceda-." Similarly: possibility, hypothesis, requests and commands trigger the subjunctive. Once you recognise these patterns your subconscience will create a reflex that will nudge you every time you say a verb that needs the subjunctive. Do not stress about all the rules, just learn them and soon you will be able to use the subjunctive automatically. I started out learning this as a second language speaker and now I don't have to think about it at all. You will reach this level too.