Although this could happen on exams, it is more likely on research papers or other work done outside of the classroom that involves research. If a sentence you give me has no quotation marks, I must assume that you are the author of those words. The goal of an analytic paper is to use sources to develop your thinking on a topic. If you merely restate what others have written, you do not show any development of your own thinking. Doing research often may be a necessary part of an assignment, but not to the exclusion of your own thinking.
Most students do their work honestly and carefully, and never run into problems with plagiarism.
Out of fairness to those students, especially since written work is a major basis for grades, I always investigate cases of possible plagiarism. Flagrant plagiarism may result in your work being counted as a zero in computing semester grades. Less flagrant plagiarism usually results in at least a failing grade for the work plagiarized. If you have questions about plagiarism or if you have any doubts about whether or not you may be plagiarizing, since it is not always intentional, discuss it with me before handing in your work. After reading the Academic Policies, please ask for any clarifications you need.