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Student Preview allows instructors to view their course as if they were a student in the course, without having to request and use a "demo student" account.
To enable Student Preview Mode, click on the iconStudent Preview Icon in the upper right. 
This icon is a grey dot circled by two arrows.
Once Student Preview Mode is enabled, you can navigate through your course just as a student would.
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When you exit Student Preview, you will see these options:
- Choose this option ("Delete the preview user and all data") if you have used Student Preview merely to see what your course looks like to a student, and you did not take any action (like take a test, submit an assignment, participate in a discussion forum, or something elsethe like).
- Use this option ("Keep the preview user and all data") if you have used Student Preview to take a test, submit an assignment, participate in a discussion forum, or taken some other action, which you would then like to see from your instructor view. In other words, if you entered Student Preview and submitted an assignment, and would then like to return to the instructor view to see the submission, choose this option.
For more information about Student Preview, see this page from Blackboard:
the Blackboard Student Preview Information help page.