It is important to remember that the subjunctive is a mood and not a tense. It is normally used to express a wish or desire or to express uncertainty. For GCSE it is normally best to learn set phrases which you know take the subjunctive- for example- no pienso que/no creo que (I don’t think that), es possible que (it’s possible that), es importante que (it’s important that), dudo que (I doubt that) and so on.