There are a number of different situations in which you use the subjunctive in French. It is a 'mood' rather than a tense, and is used in dependent clauses following 'que' or 'qu''. In French, feelings of personal doubt and desire require the subjunctive, as do expressions of necessity, possibility, and judgment. In the majority of cases when the speaker is referring to the aforementioned situations, there is no choice but to use the subjunctive in the dependent clause. The easiest way to be sure of when to use the subjunctive is to learn the verbs and set phrases that will use the subjunctive in a subordinate clause.For example:Il faut qu'il sache la vérité. It is necessary that he knows the truth.Il faut que is always be followed by the subjunctive, as it expresses necessity.Mon père veut que je fasse mes devoirs. My father wants me to do my homework.Vouloir que expresses desire, and therefore is followed by the subjunctive when the subject in the subordinate clause is different from the main clause.