Whenever you are using a "piece" of someone else's work, you would place the entire "copied section" in quotation, usually with an apostrophe. That way, your readers can tell the difference between your work and that of another. This keeps you from plagerizing someone elses work. You can also, include an author credit and attribution. Some Authors claim "Fair Use" when the are caught infringing on the works of others because they do not understand the true meaning of "Fair Use". Fair Use is mostly for Teachers who need to use a piece of someones work to teach their students, News Reporters and Authors who need to prove a point that they are trying to make. You can read about fair use in the section below that I copied from the Copyright Act:
'§ 107 . Limitations on exclusive rights: Fair use40
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.'
What is not listed above, is the usual "amount" of someones work that you can legally copy and that is five percent.