The author and the assignee can mark a task completed.
The Mark as completed check box to the left of the task name in the task list allows you to quickly change the task status to Completed and move it to the Completed tasks list. To cancel the task completion operation, click Cancel on the pop-up notification at the bottom of the pane.
You can return a task from the Completed tasks list by unchecking the Mark as uncompleted flag located to the left of the task name.
The author of the task will automatically receive the following email:
•About the task completion, once the assignee completes the task.
•About the failure to complete a task if the deadline has expired and the task has not been marked as completed.