French has two future tenses -- the futur proche and the futur simple. The futur proche is formed with the auxiliary aller which is followed by an infinitive (Je vais partir. 'I'm going to leave'). The futur simple doesn't have an auxiliary. Instead, the infinitive form becomes the stem to which future endings are added (Je partirai. 'I will leave').
The two tenses are virtually interchangeable in most contexts, especially in spoken French. So what is the difference? Traditional grammar books suggest that the futur proche is used in more informal contexts and the simple future in more formal contexts, and that the futur proche is primarily used in speech and less frequently in writing. The futur proche, also called the futur immédiat, typically refers to a time very close to the present moment, i.e, the near or immediate future. The futur simple, on the other hand, is often used for events in the more distant future.
The two tenses also indicate a difference in the speaker's perception of the future event. The futur proche indicates that the speaker is relatively certain that the future event will actually happen. In contrast, the futur simple indicates that the speaker is less certain of the future event coming to pass. Let's imagine a context to make this distinction more clear. In each example below, a woman speaks of having a baby. However, the futur proche is likely to indicate that the woman is pregnant and therefore will have a baby some day soon. The futur simple would more typically indicate that the woman plans to have a child some day in the future, but is not pregnant at present.
Je vais avoir un bébé. I am going to have a baby.
J'aurai un enfant ou deux. I will have a child or two.
In this case, the futur proche indicates both the immediacy of the future event as well as its inevitability in the mind of the speaker. Conversely, in the futur simple sentence, the time frame for the future event is left open--maybe she will get pregnant tomorrow, maybe she won't.
While considering the difference between the tenses in these two examples, keep in mind that in spoken French, there are many contexts in which either tense is perfectly suitable.
In general, the future tenses in French and in English are used similarly. However, there is one major difference in future tense usage between the two languages. French requires the future tense after certain conjunctions where English usage calls for the present tense. Let's consider these examples.
Dès que vous vous fiancerez, vous choisirez une date pour le mariage. As soon as you are engaged, you will choose a date for the wedding.
Aussitôt que vous recevrez sa lettre, vous la lirez. As soon as you get her letter, you will read it.
Quand j'aurai des vacances, j'irai voir ma famille. When I have some vacation, I will go see my family.
Lorsque les enfants sauront nager, on restera dans un village près de la mer.
When the children know how to swim, we will stay in a town by the sea.
In the French examples above, the subordinate clauses starting with dès que, aussitôt que, lorsque, and quand are in the simple future because the main clauses are in the future. The action in the subordinate clause implies future tense since it will take place at around the same time as the action in the main clause. Note the use of the present tense in the subordinate clause of the English translations.
Dès que and aussitôt que mean 'as soon as,' while lorsque and quand mean 'when.'
Exercise: 'Futur simple' or 'futur proche'? Fill in the blank with the appropriate future tense of the verb indicated in parentheses.
Quand mon frère sera riche, il ________________________________, mais pas maintenant! (se marier)
J'ai une idée pour mon essai! Je ________________________________ ce soir! (écrire)
Hélène est étudiante d'anglais. Mais un jour, elle ________________________________ professeure. (être)
C'est l'anniversaire de mon ami. Je ________________________________ un gâteau. (faire)
Lorsque Edouard verra ses amis, il les ________________________________ à sa fête. (inviter)
Aussitôt que le bébé arrivera, nous ________________________________ son nom. (savoir)
Je suis étudiant en biologie. Mais un jour, je ________________________________ médecin. (devenir = to become, conjugates like venir)
Nous avons un examen demain. On ________________________________ ce soir. (étudier)