Nick Youngson CC BY-SA 3.0 Pix4free
Everything on the internet is free to use
If I only use 10% of the content, I am not infringing on copyright law
If I don't intend to make money, it's ok to use
If I'm using it for educational purposes, it's ok under fair use
Copyright can protect my ideas
Everything without a copyright notice is not protected
This guide from the library at California State University, Long Beach, and this guide from the library at Lane Community College contain many scenarios you may encounter in your search and consideration of works to use in your teaching.
Cases involving text, art work, visual arts, audio visual internet, music and parody. A summary from the libraries at Stanford University.
On this page from copyright.gov, you will find a list of recently settled copyright cases as well as the ability to search the database by categories and legal jurisdictions.