Because a tasklist item can be a mail message, appointment, task, or note, the way you track the items as being completed can vary. The following list provides some ideas that you might find helpful:
Mail Message: Delete the message or move it to another folder.
Appointment: If you no longer want the appointment in your Calendar as well, delete it from the Tasklist; it is removed in both places. If you want to retain the appointment, move it back to your Calendar.
Task: If you want to retain the task in the Tasklist and Calendar, open the task and mark it complete (see Marking a Task Complete). If you want to remove the task from your Tasklist and Calendar, delete it.
Note: If you no longer want the note in your Calendar as well, delete it from the Tasklist; it is removed in both places. If you want to retain the note, move it back to your Calendar.