Use of future
The simple future tense is an indicative tense that generally indicates a future action. It is by extension of its basic usage that this tense can also indicate :
1) an action that will occur that is considered certain
Je vais aller à la mairie pour me faire une carte d'identité, ils sauront me renseigner.
In this example, we are sure to get an answer.
Remark: verb "aller" followed by its own infinitive is a near future: je vais aller.
2) an order. The future tense then takes on the value of an imperative and is used to indicate an order.
Pendant que je fais le ménage, tu feras la vaisselle.
Vous me nettoierez votre chambre pour demain.
"Tu feras" means "fais"and it is an order.
3) future of attenuation. The future tense is then used to ask something politely to the other person.
Je prendrai un café s'il-vous-plaît.
4) a condition. The future tense is also used to express an assumption, which then has a higher probability than when the conditional is used. It is coupled to the present tense just as the conditional is coupled to the imperfect.
Si je gagne à la loterie, j'achèterai une grande maison à la campagne. (present + future)
Si je gagnais à la loterie, j'achèterais une grande maison à la campagne. (imperfect + present conditional)
The first sentence is used when you've just bought the ticket, the second can be used when you're not even playing.
Use of past future tense
The past future is generally used in the same cases as the simple future by introducing a notion of anteriority in relation to another future or past event in relation to the main action.
An action considered to have been accomplished in a certain way:
Dans cinq minutes, j'aurai fini mon repas.
An action future but prior to another future:
Lorsque j'aurai fini ce devoir, je commencerai le suivant.