"Only one thing is impossible for God: To find any sense in any copyright law on the planet." - Mark Twain
Copyright Basics (PDF) Look over this pamphlet introducing copyright, distributed by the U.S. Copyright Office.
Copyright Crash Course An excellent resource for learning more about copyright. Georgia Harper from the University of Texas Libraries is responsible for the course content which is regularly updated and licensed under Creative Commons.
United States Copyright Office
Read the full text of U.S. federal copyright laws in Title 17 of the U.S. Code, plus the copyright, patent, and trademark regulations in Title 37 of the U.S. Code of Federal Regulations. Read online or download the PDFs.
Teach Act The Technology, Education and Copyright Harmonization Act, more commonly known as the TEACH Act, updated U.S. copyright law regarding the display and transmission of copyright protected materials by accredited, nonprofit educational institutions and is especially relevant for the use of teaching materials online.
Fair use = copyrighted material copied for a limited, "transformative" purpose: comment, criticism, parody, new interpretation, etc. Fair use does NOT require you to pay a fee or get permission from the rights holder.
The American Library Association's handy Fair Use Evaluator tool will help you decide when use is fair.
Still have unanswered questions? Try using Columbia University's Fair Use Checklist (PDF).
The public domain contains creative works that are not protected under 
U.S. copyright law and free for anyone to use. Works usually enter the public domain because the copyright has expired or the rights holder didn't renew the copyright. Stanford University offers more information.
How do I know if a work is in the public domain?
Consult Peter Hirtle's useful chart illustrating Copyright and the Public Domain in the United States (2020).
Why is the public domain valuable? Stanford University tackles Public Domain.
https://fairuse.stanford.edu/overview/public-domain/welcome/