How to know

I've prepared a Fair Use Cheat Sheet that you can consult. I suggest you do so.

Fair Use rationale

​Are  you remixing/repurposing existing works? Does your montage incorporate copyright-protected video? If so, you need to submit a written Fair Use Rationale to Mr. Huette. This page explains that assignment. Your rationale needs to be typed, and it should answer each of the following four questions in as much detail as possible: 
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1. What kind of copyright-protected content are you using? Where did it originally come from? Who is the copyright holder? 
2. How much of the original content are you using? Is the amount that you used “appropriate” in your view? How did do arrive at this conclusion? 
3. To what extent will your work substitute for the original? Does it occupy the same market? Could someone watch yours instead of watching the copyright-protected original? 
4. In what way have you transformed the content from its original form? How is the message different from its original intent? ​