Hey @Mickael_Churet, Michael here from Asana
@Julien_RENAUD is right on the money!
Asana doesn’t (& wouldn’t!) delete your Task, unless you specifically select the option to delete it. The system merely removes it from view and classifies if as “Completed”. This way your list of Tasks left to do becomes less cluttered and allows you to focus on the work you have left to complete.
You can toggle the sort view filter button at the top right of the List to view either Incomplete, Complete or All Tasks to view all the Tasks in your Project (in one way or another).
Hope this helps!