Each course contains several units (Introduction, units 1-4, Summary). When you buy a course the first unit is available or open. List of all possible unit statuses: awaits, available, open, completed, closed. You will find information about the unit status and date of publication of course materials in courses after logging in. Until the unit is being published it awaits, after the unit has been published but before it begins it is available. Once the submission of exercieses is possible the unit is open. After completing all required exercises the unit is completed and after the course has finished the unit changes its status to closed.