When some tasks are completed, you might want to mark them as completed so you do not need to worry about them anymore. Please see the steps below:
Step 1: Click the "Tasks" button to open the Tasks window;
Step 2: Select the task that you want to mark as completed;
Step 3: Click "Mark Complete" from the "Home" tab;
Step 4: The task will be marked as completed with a line through it (please see how to select a color for completed tasks);
Alternatively, check the box next to the name of the task you want to mark as complete in the "Tasks" window.