Hi Kai.
I'm not sure, but I _think_ that you are asking how to make completed tasks get cleared out of your outline. You can do this by going to the Tools menu, selecting Options, then AutoArchive. Assuming that you do not need to keep a copy of each completed task, I would suggest checking the forst checkbox (run autoarchive every . . .) and setting the interval to one day. Then, I would suggest typing "1 day" in the space by "completed more than _____ ago". Also, click the button nexty to "permanently delete old tasks. This will cause every task to disappear between two to three days after you complete it.
Do you want the completed tasks to be hidden from view for the days they are waiting to be archived? If so, look for a button in the top toolbar that has an eyeball with a checkmark in it. Click it until it shows a white eyeball with a check, and the label "completed tasks hidden."
If this is not what you were looking for, please come back and try explaining what else you need.
-Dwight