Bonjour :) La manière la plus simple de répondre à cette question : You can use "Je mangerai" when the action will be far future and it has no impact on the present. You can use "Je vais manger" when the action is situated in a near future. The action has an impact on the time being. The same for "Je mangeais" : you use it when the action is situated in a distant past. On the contrary, "J'ai mangé" means that the action is situated in a near past. You just have eaten. I hope I am clear... Voilà :)