The subjunctive is required after a specific, learnable set of expressions in the main clause, most falling into four categories: wish or will (vouloir que, souhaiter que), emotion (être content que, avoir peur que), doubt or possibility (douter que, il est possible que), and necessity or obligation (il faut que, il est nécessaire que).
Je veux que tu viennes (I want you to come), j'ai peur qu'il pleuve (I'm afraid it will rain), and il faut que nous partions (we have to leave) all use the subjunctive because of the trigger phrase, not because of anything about when the action happens. If there's no que and no change of subject, French uses an infinitive instead: je veux partir (I want to leave), not je veux que je parte.