Copy-and-paste is one of the most common forms of plagiarism and is the style used by plagiarizers. However, even honest writers make this mistake due to improper or lack of referencing of the source material. According to Princeton University's article on examples of plagiarism, a significant passage should be quoted entirely if the point is made clearly and concisely. In such cases, the writer can copy the passage word for word and clearly attribute the passage to the source material.
Word-switch plagiarism is a close variant to copy-and-paste. The concept of lifting another author's words without proper sourcing remains the same except that certain words are added or replaced in the sentence. This is still considered plagiarism because the main idea and structure of that sentence is still the author's and needs to be credited as such.
Princeton University notes that even if an article is paraphrased enough to the point where none of the original text can be found, it would still be considered plagiarism. The writer would have still copied the original author's style using the same essential outline, order of examples and fundamental main points. Writers should credit the idea that belongs to the original author within the text and refer to the original article in the bibliography.
Referencing the same source repeatedly in an article may seem counter-intuitive and unnecessarily wordy for some writers. However, in the University of Sussex's article on types of plagiarism, any substantially different text that does not belong to the writer must go back to the source -- even if that source has already been credited. For example, writers will have to source "properties of metal elements" and "common chemical compounds" separately even if both articles are contained in the same chemistry book.
The main idea behind a piece of writing rightfully belongs to the author and must be credited as such. However, some students may mistake an author's idea as common knowledge. Common knowledge is composed by facts, opinions and ideas that are typically free or taken as common sense by general society. Princeton University's Publication, "Academic Integrity at Princeton (2008)" on common knowledge gives a guideline for writers: "...if you can find the information in The World Book Encyclopedia, then it's 'common knowledge'." For example, explaining the process of basic mathematical addition in an article does not need to be referenced to any particular source.