Under the “fair use” rule of copyright law, an author may make limited use of another author’s work without asking permission. However, “fair use” is open to interpretation. Fair use is intended to support teaching, research, and scholarship, but educational purpose alone does not make every use of a work fair. It is always important to analyze how you are going use a particular work against the following four factors of fair use.
U.S. Copyright Office provides a fact sheet
University of Texas provides a summary of Fair Use.
The following links can provide helpful information on deciding if you are using copyrighted material fairly: