Two ways on how on completing a task are as follows:
Outside Task Properties
Click on the empty check box just before the task name to complete the task.
Once a task is completed, it will appear in the Completed Tasks section at the bottom of the Task List.
Inside Task Properties
Click on the Task to open Task Properties.
Click on the empty check box just before the task name to complete the task.
Once a task is completed, it will appear in the Completed Tasks section at the bottom of the Task List.