The subjunctive is hard even for native speakers, so don't worry! There are a few handy rules to remember. Use the subjunctive in Spanish:
1. If the action you are describing did not actually happen: "Si hubiera estudiado, hubiera aprobado el examen." (If I had studied, I would have passed the exam.)
2. If you talk about actions performed by two different people in a sentence: "Quiero (yo) que hables (tú) sobre tu trabajo." (I want you to talk about your work.)
3. If you give a negative command, i.e. tell someone NOT to do something: "¡No hables conmigo de esa manera!" (Don't talk to me like that!)
4. If you are using certain set phrases expressing desire: "Ojalá venga Jaime a la fiesta." (I hope Jaime comes to the party.)
5. If you are talking about a future action: "Cuando esté en la universidad, seguiré estudiando el español." (When I am in university, I will continue studying Spanish.)
As you can see, the subjunctive is often used in situations where an action is hypothetical, and the above rules cover the majority of situations where we need to use this grammatical "mood". It seems daunting because we don't use the subjunctive in English, but it's not as hard as it seems at first! Try listening out to native speakers and hear when they use it too!