I have found that many students are confused and helpless when it comes to understanding why a professor, including myself, has accused them of plagiarism. It seems to be a common practice for student writers to find an online article and then copy and paste a sentence into their paper. They often put a sentence from one article in the same paragraph as another one. Sometimes the original articles are cited, but more often not. When we sit down in my office to talk about it, they tell me this was how they wrote all their papers in high school. Some students from outside the US have told both Harvey and me this is how they were taught to write papers.
But let me make it clear. In the US, at university and professional levels, scholarly writers are expected to generate their own words and not copy any strings of words from sources. If you do lift a sentence or phrase, it must be in quotation marks, with the source properly cited, with page numbers. There is only one reason to use a sentence verbatim from a source: The author of that work says something really important to your argument and there simply is no better way to say it.
If you do use ideas from a source, you must cite the ideas you use, not just direct quotes. Basically you must cite any time the question arises in the reader’s mind: How do you know that?
Merely assembling sentences from online websites and PDFs of articles and books is not writing. It seems like some kind of word salad. The writing you produce comes from your own thoughts and opinions, not a goulash of other people’s ideas.
P.S. Most professors at American colleges and universities have access to software that checks student assignments against what’s on the Internet and elsewhere, so the chances of getting caught for plagiarism are much greater than ever.