After marking a task as completed, it disappers.
How can I view it in the future?
You can click on your karma points and select "View All Completed". You should be able to see your completed tasks and filter them by date, label, or project.
Another way to do it is by "Showing completed tasks" in a project by clicking on the icon that's directly below the lower right corner of the last tasks.
Lastly, you can use keywords to search for a particular tasks. If it finds a match in your completed history, you should be able to find it.