Copyright does not last forever. The main purpose of copyright law is to allow for creators of works to be reasonably rewarded for their creative efforts. To that end, there are statutory rules to determine when copyright protection of a work comes to an end (e.g., the "life plus 50” rule). When the term of copyright expires, the work is said to come into the "public domain" and is then available for anyone to use and copy without seeking permission from the copyright owner (the author retains no rights in the work). This is the reason that Emily Carr's paintings and Stephen Leacock's writings no longer are protected by copyright.
Although the copyright in Shakespeare’s plays expired long ago, many of the published editions of his plays contain added original materials (such as footnotes, prefaces etc.), which are copyright protected because the authors have used skill and judgment in creating the new material. This creates a new copyright in the added original material, but not in the underlying text of the original work in which the copyright had expired.
Don’t assume that everything you find on the Internet is in the public domain just because it is publicly available. Most of the material you find online is protected by copyright; however, you may be able to use it for educational purposes because fair dealing or the exception for educational use of material publicly available through the Internet will cover many uses related to teaching. See the FAQs under Instructors – Using copyright protected material from the Internet for further information about using material found on websites.
Works can also be in the public domain because the work was either not eligible for copyright protection in the first place or the copyright owner has given copyright in the work to the public. Some copyright owners have made clear declarations that uses of their copyright works may be made without permission or payment. Such a declaration may be found on the work itself or the website where it is found. The Reproduction of Federal Law Order, for example, permits anyone, without charge or request for permission, to reproduce Canadian laws and decisions of federally constituted courts and administrative tribunals in Canada. However, restrictions can be placed on the uses that can be made of works and, in this case, you must be sure to use the material accordingly.
Project Gutenberg is a good resource for finding literary works in the public domain. See the Instructors' resources for more suggestions about how to locate works in the public domain.
For more information about duration of copyright protection in Canada see the Government of Canada’s About Copyright publication and the Canadian Public Domain Flowchart (by the Copyright Office at the University of Alberta (2020), licensed CC BY).