On the Tasks page of a workspace, you will find the main task list (incomplete tasks), Backburner and Completed list.
On your My Work view, you will see all your incomplete task groups and tasks across workspaces in a single list and all your completed task groups and tasks in a list below.
Completed Items are in the Completed List on the Tasks Page
When you click the checkbox beside a Task Group, Task or a Backburner item, the item will move to the Completed list, which is at the bottom of the Task page. This reduces the clutter on your Task page so you can focus on what needs to be done.
How do I move the completed item back to the main list?
Just uncheck an item and it will move back to its original location.