The subjunctive mood is used to express actions or ideas which are subjective or otherwise uncertain: will/wanting, emotion, doubt, possibility, necessity, judgment. It is nearly always found in dependent clauses introduced by que or qui, and the subjects of the dependent and main clauses are usually different.
Je veux que tu le fasses.
I want you to do it.
Il faut que nous partions.
it is necessary that we leave.
The following pages of this lesson include lists of verbs, expressions, and conjunctions which require the subjunctive in French. They are divided into categories to help you remember them, but if you're ever in doubt as to whether any given expression needs the subjunctive, my Subjunctivator can give you a quick yes or no.
The subjunctive can seem overwhelming, but the thing to remember is the subjunctive = subjectivity, unreality. That should help you figure it out at least 90% of the time.
NOTE: There is no future subjunctive. Even if the action is to happen in the future, the present subjunctive is used. However, there is a past subjunctive.
Quiz: Subjunctive or indicative?