Ohh boy haha ... this is a complicated question that would probably best be answered for you by a textbook and not me, but I'll try and sum it up: the subjunctive is a mood, not a tense. It is used to express doubt, hopes, fears, recommendations, probability and possibility. There are also certain phrases that ALWAYS require a following subjunctive clause, like "para que", "sin que", etc. Also there are constructions in spanish where the subjunctive is used, like "si yo ahorre (subjunctive) más dinero, viajaré (future) a Europa ..." ("si" meaning if and therefore professing a probability that isn't actually certain or real). The subjunctive is one of the many rich layers of language, and to truly understand all of its intracies and rules I would suggest to go browse your local borders because there are whole books devoted to its usage in spanish haha. Having formally studied spanish for over 6 years in school and the university, it truly takes time and just plain old memorization to understand when and how the subjunctive is supposed to be used. I recommend personally "The Ultimate Spanish Review and Practice". It is a very good/cheap book for intermediate to advanced learners, and their section on the subjunctive is very thorough.

el modo subjuntivo is a gramatical variety only for verbs in spanish its used to express doubt, wish or probability. subjunctive sentences often are followings with other sentences examples: yo creo que ellos no llegan hoy mi jefe quiere que trabaje en domingo quiza mañana llueva temprano yo quiero que gane españa