Plagiarism is especially tempting in academic essays. Quotations are essential, but you might be tempted to quote someone else's material and present it as your own. It is very easy to do this, but it is very easy to get caught. You won't get caught if you don't do it. In every case, you are better off adding quotation marks. Three or four unquoted words in a row can be enough for you to get charged with plagiarism.
Here are five signs that you may have plagiarized. (There are other signs too, and experienced readers know to look for them. Suspicious sentences will send them straight to the Internet.) If you notice one of these signs, try to find the original source. If you cannot find it but it is essential to the essay, it is best to add quotation marks and to note in the essay that you can no longer find the source.
(1) An uneven style. This sign mainly refers to a well-written sentence or paragraph in the midst of badly written sentences and paragraphs. When you move from your rough draft to an edited draft, you might find an eloquent sentence that has the ring of polished, previously published prose. It probably came from someone else. This sign also refers to vocabulary that shifts from basic words to advanced words from one section to the next.
(2) An especially long sentence in the midst of many short sentences. Published essays tend to have longer sentences than academic essays written by students.
(3) A paragraph or group of sentences that seem to be proving a point very well, but not the point that you advertised in the essay. When an entire essay does not adequately address the assigned prompt, the whole essay is suspect. Similarly, if you advertise that a paragraph is about A but there are several sentences about B that seem to belong better in a different essay, the paragraph will be suspect. Sometimes this is a matter of restructuring the essay, but sometimes it is plagiarism.
(4) A level of sophistication or communication that exceeds the normal level for you and your peers. If you are in high school and you write a history essay that uses primary sources not discussed in class--without citing a secondary source--your reader will wonder how you possibly got the data. If you are in an introductory humanities course and you already seem to know several critics' opinions of the text--in contrast to your performance in class--your reader will wonder how you became so well read so quickly.
(5) Passages that use different formatting: font, point size, number of spaces after a period, color, etc. You might be surprised how many plagiarists get caught by an entire paragraph being printed in gray instead of black, or by a superscript number that used to refer to a footnote in an essay with no footnotes. Also, copying and pasting material from web sites often produces a lot of "nonbreaking space" characters, which look different on the screen from spaces that are typed directly; many plagiarizers get caught when they are asked to produce an electronic copy of their essay.