The subjunctive tense is probably the trickiest element of Spanish A level grammar, which is why demonstrating that you know how to use it correctly in assessments, will edge you into that top mark band.
In general, the subjunctive mood is used to describe something that subjective and possible. Something that is not certain.
It is used to talk about doubts, wishes, the abstract, emotions, and other unknown and non-factual or hypothetical situations. It's mostly used to make recommendations, give commands, and talk about how things make you feel. It is also used to express opinions about another action.
I find it useful to use the acronym WEIRDO to determine whether or not I need to use the subjunctive tense. (Wishes, Emotion, Impersonal expression, Recommendation, Doubt and 'Ojalá.')
After you've understood these rules, the best way to master the subjunctive is to practice using it! I am more than happy to go through practice sentences with you - soon it will become second nature!