After you’ve finished a task, you can easily mark it as completed. That will remove the task from the active task list and show you that you’re crossing items off your list.
Click to select a task.
On the Home tab, in the Manage Task group, click Mark Complete.
Alternatively, right-click a task and then choose Mark Complete.
You can also select the check box to the left of the task to mark the task as complete in the task window.