When does il faut take the subjunctive in French?

il faut takes the subjunctive whenever you name the person who has to act, because naming them forces a que clause: il faut que tu partes (you must leave), il faut que nous fassions attention (we must be careful). If you do not name a specific subject, il faut simply takes an infinitive and no subjunctive: il faut partir (one must leave / we need to leave). So the choice is not really about il faut at all - it is the different-subject rule. A new subject after que pulls in the subjunctive every time.

Why is it 'je veux partir' but 'je veux que tu partes'?

Because of the different-subject rule. When the subject of vouloir is also the subject of the second verb - I want, I leave - French refuses to repeat the subject and uses a plain infinitive: je veux partir (I want to leave). When the two subjects differ - I want, you leave - French cannot use the infinitive, so it opens a que clause and puts the second verb in the subjunctive: je veux que tu partes (I want you to leave). English uses 'you to leave' in both registers; French strictly separates the same-subject infinitive from the different-subject subjunctive.

Which expressions of necessity and will trigger the subjunctive?

The necessity set: il faut que, il vaut mieux que (it is better that), il est necessaire que, il est important que, il est essentiel que. The will-and-wanting set: vouloir que (to want that), souhaiter que (to wish that), desirer que (to desire that), exiger que (to demand that), demander que (to ask that), and ordonner que (to order that). All of them describe something not yet real - something wanted, required or demanded rather than stated as fact - which is the underlying logic of the subjunctive. Contrast verbs of certainty like penser que or il est certain que, which take the indicative.

The French Subjunctive After Necessity and Will

If you have learned how to build the present subjunctive on the formation page, this is where you learn the first reason to actually use it. The subjunctive never appears on its own; something has to trigger it. The easiest and most common trigger group is necessity and will - the language of must, want, demand and wish. Start here, because these expressions turn up in nearly every conversation and the logic behind them carries over to the harder triggers later.

The thread running through the whole group is this: necessity and will describe things that are not yet real. When you say you want someone to leave, they have not left. When you say something must happen, it has not happened. French marks that unreal, willed quality with the subjunctive. Plain statements of fact stay in the indicative; wanted, required and demanded things go to the subjunctive.

il faut que: the workhorse

il faut que means "it is necessary that" and is the single most common subjunctive trigger in French. Name the person who has to act, and the verb after que goes subjunctive:

Il faut que tu partes. (You must leave. / You have to leave.)
Il faut que nous fassions attention. (We must be careful.)
Il faut que vous soyez a l'heure. (You must be on time.)
Il faut qu'elle finisse son travail. (She has to finish her work.)

Note partes, fassions, soyez, finisse - all subjunctive, and faire, etre pulling out their irregular forms from the formation page.

If you do not name a specific person, il faut takes a plain infinitive and there is no subjunctive at all:

Il faut partir. (We need to leave. / One must leave.)
Il faut faire attention. (One must be careful.)

That contrast - il faut partir versus il faut que tu partes - is the different-subject rule in miniature, and it governs the entire group.

The different-subject rule

This is the rule to engrave. It decides infinitive versus subjunctive for every verb on this page.

Same subject on both verbs -> infinitive, no que.
Different subjects -> que + subjunctive.

Watch it with vouloir:

Je veux partir. (I want to leave.) - I want, I leave: one subject, so infinitive.
Je veux que tu partes. (I want you to leave.) - I want, you leave: two subjects, so que + subjunctive.

English hides this: "I want to leave" and "I want you to leave" look structurally similar. French treats them as two completely different builds. You can never say "je veux que je parte" - if the subject is the same, the infinitive is compulsory.

English	Subjects	French build
I want to leave	I / I	je veux partir
I want you to leave	I / you	je veux que tu partes
She wants to come	she / she	elle veut venir
She wants us to come	she / we	elle veut que nous venions

vouloir, souhaiter, exiger, demander: verbs of will

A cluster of verbs expressing wanting, wishing and demanding all behave like vouloir. With a different subject they take que + subjunctive:

Je veux que tu viennes. (I want you to come.)
Elle souhaite que nous soyons heureux. (She wishes us to be happy.)
Le professeur exige que les eleves fassent leurs devoirs. (The teacher demands that the pupils do their homework.)
Il demande que vous attendiez ici. (He asks that you wait here.)
Nous desirons que tout se passe bien. (We wish for everything to go well.)

Common members of the set: vouloir que, souhaiter que (to wish), desirer que (to desire), exiger que (to demand), demander que (to ask), ordonner que (to order), preferer que (to prefer). All of them describe an act of will aimed at someone else, which is why the different-subject clause goes subjunctive.

A note on irregular forms: when the second verb is aller, venir, etre, avoir, faire and the like, it takes its irregular subjunctive - que tu ailles, que nous venions, que vous soyez, qu'il fasse. If any of those look unfamiliar, the full tables are on the formation page.

il vaut mieux que, il est necessaire que, il est important que

Impersonal expressions of necessity and advisability behave like il faut que - with a named subject after que, the verb is subjunctive:

Il vaut mieux que tu restes ici. (It is better that you stay here.)
Il est necessaire que nous partions tot. (It is necessary that we leave early.)
Il est important que vous compreniez la regle. (It is important that you understand the rule.)
Il est essentiel qu'elle sache la verite. (It is essential that she knows the truth.)

And again, with no specific subject these revert to de + infinitive: il vaut mieux partir (it is better to leave), il est important de comprendre (it is important to understand). Same different-subject logic, every time.

A quick contrast: certainty takes the indicative

To stop the subjunctive spreading where it does not belong, hold one contrast in mind. Necessity and will are unreal, so they take the subjunctive. Certainty is factual, so it takes the indicative:

Je veux qu'il vienne. (I want him to come.) - will -> subjunctive vienne.
Je sais qu'il vient. (I know he is coming.) - certainty -> indicative vient.

Verbs like savoir que, penser que (affirmative), il est certain que and il est vrai que state facts and stay in the indicative. The detail of doubt, opinion and the negative-and-question flips belongs to a later page; for now, just note that necessity and will trigger the subjunctive, plain certainty does not.

Worked examples

Il faut que tu fasses tes devoirs. (You have to do your homework.) - il faut que + irregular faire.
Je veux que vous soyez prets a huit heures. (I want you to be ready at eight.) - vouloir que + irregular etre.
Elle souhaite que nous venions avec elle. (She wishes us to come with her.) - souhaiter que + two-stem venir.
Le patron exige que tout le monde arrive a l'heure. (The boss demands everyone arrive on time.) - exiger que + regular arriver.
Il vaut mieux que nous partions maintenant. (It is better that we leave now.) - il vaut mieux que + partir.
Il est important qu'il aille chez le medecin. (It is important that he goes to the doctor.) - il est important que + irregular aller.
Je veux partir tot. (I want to leave early.) - same subject, so infinitive, no subjunctive.

Common mistakes English speakers make

The classic error is breaking the different-subject rule: French speakers never say "je veux que je parte" - if the subject is the same, you must use the infinitive, je veux partir. Conversely, do not smuggle an infinitive in where the subjects differ: "je veux toi partir" is not French; it has to be je veux que tu partes. Another trap is leaving the second verb in the indicative after a trigger - it is il faut que tu partes, not "il faut que tu pars". Watch the irregulars too: "il faut que tu es" is wrong; etre goes subjunctive, il faut que tu sois. Learners also over-apply the rule, putting a subjunctive after verbs of certainty - je pense qu'il vient, not "qu'il vienne", because thinking something is true is a statement of fact. Finally, remember that the bare impersonal forms drop the subjunctive entirely: il faut partir and il est important de comprendre use the infinitive because no specific subject is named.