Copyright duration varies from nation to nation:
In Berne-signatory countries, the duration of copyright is dependent on the duration of the author's life. Berne requires that copyright exist until a minimum of 50 years after the author's death. A number of countries, including the European Union and the United States, have extended that to 70 years after the author's death. A small number of countries have extended copyright further. The current lengthiest term is 100 years after the death of the author (Mexico).
https://en.m.wikipedia.org/wiki/List_of ... ght_length
Also worth considering is fair use, the reproduction of portions of a work for specific purposes, which include teaching:
https://en.m.wikipedia.org/wiki/Fair_use
Unfortunately, while copyright duration is quantitative and therefore easy to interpret for applicability, fair use is qualitative and infringement claims can be murky to predict when fair use is the defense.
Except in cases where the fair use falls safely within established guidelines, when in doubt it may be useful to seek permission from the author or the author's estate, or bypass the issue by creating one's own original work.
As a practical matter, copyright and patents can be difficult and expensive to pursue claims of infringement in court, and they can be equally expensive to defend. This means fewer lawsuits than potential infringements, but it also means that if it comes to that litigation can be devastating to a small business. Indeed, patent trolls rely on that to collect settlements, but that's another story.
For myself, I try to steer clear of any potential infringement, for my own peace of mind and that of any entity which may later acquire my property.
That said, I am not an attorney, and am obliged to note that nothing I've said here can be construed as legal advice, and to encourage questions requiring legal advice to a licensed professional operating within the relevant jurisdiction.
