Within Teamwork.com, a task list is automatically marked as complete once each of the individual tasks in the list have been completed.
In Table or List view, click the checkmark to the left of each active task in the list to complete it. Once the last active task in the list is completed, the task list itself will automatically be marked as complete.
The task list will be moved to the Completed Task Lists section of the Task Lists panel on the left of both List and Table view.
Note: If the task list is pinned when the last active task is completed, the task list itself will remain open.
For more information, see: Pinning a Task List Open