A task can be marked as completed by either its author or its assignee.
To mark a task as completed, check the box to the left of the task name in the task list. The task status will change to Completed and the task will move to the Completed list.
At the same time, a notification will appear at the bottom of the Taskbar. If you need to cancel the operation, click Cancel. The task will return to the list of uncompleted tasks.

It is possible to return the task from the Completed list by clearing the box to the left of the task name.
The task author will automatically receive an email:
–On the completion of the task, when the assignee marked the task as completed.
–About non-completion of the task, if the deadline has expired and the task has not been marked as completed.