Common knowledge and paraphrasing
The only source material that you can use without attribution is material that is considered common knowledge and is therefore not attributable to one source. Common knowledge is information generally known to an educated reader, such as widely known facts and dates, and, more rarely, ideas or language. Facts, ideas, and language that are distinct and unique products of a particular individual's work do not count as common knowledge and must always be cited. Figuring out whether something is common knowledge can be tricky, and it's always better to cite a source if you're not sure whether the information or idea is common knowledge you have encountered the information in multiple sources but still think you should cite it, cite the source you used that you think is most reliable, or the one that has shaped your thinking the most. Widely known scientific and historical facts—such as the molecular structure of water (H2O), generally count as common knowledge. You can include such facts in your writing without citation and without fear of committing plagiarism. On the other hand, as soon as your discussion becomes more specific and puts forth assertions that would be the product of an individual's thought, research, or analysis, you do have to cite.
Paraphrasing the work of others by altering a few words and changing their order or by closely following the structure of their argument, is plagiarism because you are deriving your words and ideas from their work without giving due acknowledgement. Even if you include a reference to the original author in your own text you are still creating a misleading impression that the paraphrased wording is entirely your own. It is better to write a brief summary of the author’s overall argument in your own words than to paraphrase particular sections of his or her writing. This will ensure you have a genuine grasp of the argument and will avoid the difficulty of paraphrasing without plagiarising. You must also properly attribute all material you derive from lectures.
Sources: University of Oxford, Harvard University and Delft University of Technology