First, let us be clear about the subject. Plagiarism is not synonymous with copyright infringement. You can plagiarize a work that is not subject to copyright protection, and you can infringe on a copyright even when not plagiarizing it.
Copyright is only awarded to a work during the lifetime of the author and seventy (70) years thereafter. During that time, with the exceptions spelled out in the fair use doctrine (a.k.a. fair dealing in Europe), you may not publish the work without the permission of the author. Even properly citing the work to the original authorized publication or website does not cure the infringement.
Once the copyright period has expired the work enters the public domain and is no longer subject to copyright protection. If you set out a poem that has entered the public domain, such as a sonnet of Shakespear, you must still properly credit it to the original author, or you will commit plagiarism: the usurping of the work of another person as it were your own.
Fair use is a limitation of the copyright restrictions as codified at section 107 of the copyright law (title 17, U. S. Code):
"The purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes."
Use of copyrighted work for criticism or review is specifically allowed in the copyright law. Derivative works, such as parody or satire are exempt, but only if presented in that light. Derivative works that develop the concepts, rather than regurgitating them, are exempt. Use of recognizable characters created by another is infringement. Copying works for the students in a class is exempt, but publication in a text book requires permission. Use of a copyrighted work, not in parody, for profit, without permission is piracy.
"The nature of the copyrighted work."
You cannot copyright facts, such as shocking events. For example, in 1992 all three network channels aired quick takes on the Long Island Lolita, yet neither Amy Fisher nor Joey Buttafuoco gave permission or were compensated.
On the other hand, a work of fiction or poetic expression is de facto protected.
"The amount and substantiality of the portion used in relation to the copyrighted work as a whole."
There is no hard and fast rule. A biography of George Washington was usurped and successfully halted where it used a couple hundred pages of the original work. The determining issue is not a strict percentage or quantification; rather it is whether the offensive work substantially takes the essence of the work.
"The effect of the use upon the potential market for, or value of, the copyrighted work."
This one is relatively simple: does the offending work divert compensation that should have gone to the original author, or does it diminish the market for the work.