Sometimes people who discover that you have used their work will try to get money from you by claiming you have breached their copyright. They might threaten you with a lawsuit in the hope that it will scare you into paying them something. They may do this even if they never intend to go to court.
The first thing to do is stay calm and find out a bit more about what the law says.
You are allowed to “fair use” of other people’s material
Scenario one: You coach the local junior baseball team. You decide to teach some basic sports science, believing that understanding more about their body will help them improve their fitness and results. You photocopy a bunch of pages from a textbook you own and give them to the kids.
While the book’s author owns the copyright to the material within it, a court would probably dismiss their claim that you have breached their copyright as nonsense. It would likely consider that your photocopying qualifies as fair use.
Scenario two: Your classes are such a success that you decide to try and make some money. You take the material you photocopied, incorporate it into a manual and brand it as your own. Then you start promoting it to junior league teams around the country for $500 a pop, as the secret they need to improve their performance. If the author took you to court now, a judge would likely be more sympathetic to their claim.
Whether or not something qualifies as fair use or a breach of copyright comes down to a judge’s discretion. They would typically look at the quantity and frequency of reproduction, what you used it for, whether you profited from it, and whether the copyright owner lost out because of it.
If someone accuses you of infringing their copyright, get legal help to see where you stand so you can make appropriate decisions on what to do next.