Intentional and unintentional plagiarism
The recent sixth edition of the MLA Handbook includes a chapter on plagiarism and makes the point that plagiarism can be unintentional: "as when an elementary school pupil, assigned to do a report on a certain topic, goes home and copies down, word for word, everything on the subject in an encyclopedia" (69-70).
Even beyond elementary school, students may fail to understand how to quote accurately and paraphrase effectively, and as indicated by the right-hand list below, it is possible for students to plagiarism without realizing they are doing so.
Whether a teacher judges an instance of plagiarism as intentional or not depends on three factors: the age of the student, the nature of the offense, and the scope of the offense.
Age of the student
A freshman or sophomore with little research experience might argue successfully that poor paraphrasing (for example) was unintentional--the student simply did not know better. A junior or senior who has completed several research assignments should know better, and for such students, carelessness or hastiness does not excuse plagiarism.
Nature of the offense
It is one thing to include a word used by an author without understanding what it means or to paraphrase inadequately so that a paragraph sounds too much like the original. It is another to insert whole chunks of an author's work into an essay without quotation marks.
Scope of the offense
One passage that is poorly paraphrased in an otherwise meticulous essay, or one citation that is missing, or even one short quotation that is not enclosed in quotation marks--these are a far cry from an essay that is packed with such errors.
To generalize, a teacher would judge as unintentional the plagiarism of a younger student committing any of the errors listed below on the right a handful of times in an essay A teacher would judge as intentional the plagiarism of an older student committing such errors throughout an essay. The point, of course, is not to embarrass or punish any student; it is to prepare all students for the rigorous standards of American colleges, which assume that students understand plagiarism and which treat all cases as intentional.