I like to be able to create a filter to give me all completed tasks. The point is I don't know what arguments to use when i define a filter. Thanks in advance for your answers.
Unfortunately, that's not possible, at least not through search. To see the full list of your completed tasks, please click on the Karma score and then "View All Completed".
Here, you can use 3 drop-down menus to filter this list by timeframe, project and label.
Actually an opportunity to include completed task into a filter view is an extremely desirable feature. For instance, filtering tasks assigned to a particular person for a particular period of time, I'd like to see which of them they have already completed.
Please, consider this option for future updates.