Most students of French (myself included in my days of study) fear! — no, don’t understand! – no, intensely dislike! – the verb tense, the subjunctive, and avoid it like the plague! Oui, it is another formation of verbs to memorize, and its usage can be a little complicated: I will try to simplify it once and for all!
Subjunctive: Grammar denoting a mood of verbs used when the content of the clause is being doubted, supposed, feared true, etc., rather than being asserted. (Source: The Free Dictionary)
Requires the subjunctive:
1. Two different subjects in two clauses of the sentence
EX: I want you to do it. = I want (that) you do it. = Je veux que tu le fasses.
2. follows ONLY the conjunction “que” or “qui”
EX: I am happy you are going. = I am happy (that) you are going. = Je suis contente que tu ailles.
3. The expression of an wish, emotion, feeling, doubt/uncertainty, fear, regret, necessity (Il faut que..) in the main clause (as opposed to stating something sure or perceived as certain)
EX: I don’t think she is coming. = I don’t think that she is coming. = Je ne crois pas qu’elle vienne.
4. With certain expressions (+ que) that convey a subjective idea (these require memorizing)
EX: Il faut, il vaut mieux, il est important, il est dommage, avant que, just to name a few.
Once you learn the verb conjugations, of which there are many irregular verbs, using this tense is a matter of memorization and practice – but if in DOUBT, use the SUBJUNCTIVE! (fun interactive quiz with this link)