How to deactivate task?

Users can deactivate tasks in the task edit form. Deactivated tasks will not appear on the "Today" tab, but all task data (including all task entries and notes) are kept. Users can re-activate it later and keep tracking the task.

For example, after finishing studying "English" for the semester, the user can deactivate the task for some months and activate it again when there is an English class.