课程开始：表示课程已经开始开放课程资源提供同学学习。
Course start：Course resources are available for student to learn.
课程结束：表示课程已经不允许报名学习，在课程结束之前报名的用户，（大多数课程）可以继续让用户在课程内学习
若用户在课程结束之前并未报名，则只能等待下一期开课。
Course end：You can't enroll the course anymore.
If you enrolled the course before it closed, you can still use the resources in the course;
if not, please wait for the course opening again.