Within the current copyright law is a codified concept called "fair use" (Section 107 of Copyright Law). Fair use lets someone other than the copyright holder copy and distribute copyrighted material - in certain situations -without first getting permission. Without "fair use", copyright's avowed constitutional purpose to enhance learning, and promote knowledge, wouldn't work. The law specifically allows fair use for criticism, comment, news reporting, teaching, and scholarship or research.Here are the four guiding factors to be considered (and all 4 factors are considered together, not just one, or some, of them):
1. The purpose and character of the use, including whether that use is of a commercial nature or is for nonprofit educational purposes. Also, how transformed the work is by your adding new expression or meaning makes a difference. In other words, if you are creating a parody, you are using the work to critique it, or you have added value in terms of new insights and understandings, then you are probably not in violation of fair use.
2. The nature of the copyrighted work. Nonfiction, published works (because the dissemination of facts or information benefits the public) are more usable under fair use than an unpublished PowerPoint or a fiction story.
3. The amount and substantiality of the portion used in relation to the copyrighted work as a whole. That is, how much material was taken from the entire work? The more someone takes, the less likely that the use is fair. Also, there is a qualitative measurement: if you borrow "the heart" of the work, even though it is not much, it may still be too much.
4. The marketability factor: The effect of the use upon the potential market for or value of the copyrighted work. That is, how was the material used? If the work was significantly creatively changed, it is more likely to be considered "fair use." A good question to ask yourself: Will my copying this item result in the copyright holder earning less money?
Here is a marketability example: If copying a script of a play for students means that the owner of the rights to the play would not get royalties, that is a violation of copyright.
**Here, for consultation purposes, is checklist from Columbia University Libraries to help you decide: http://copyright.columbia.edu/copyright/fair-use/fair-use-checklist/