Instructors often suspect plagiarism when they note obvious changes in the quality of a student's work (such as style, vocabulary, and content). These changes serve as an indicator that a student might have plagiarized, either intentionally or unintentionally. For example, students might attempt to paraphrase to convey information obtained from research but fail to cite their sources. Or they might fail to identify passages as quotations when they are conducting research for a writing project and later treat the passage as though it were a paraphrase. In still other cases, students will knowingly attempt to pass off the work of other writers as their own.
Students often feel the need to cheat or plagiarize when they don't understand an assignment or concept or when they don't have the time to adequately prepare for turning in an assignment (Dornan et al. 145). Offering ample time in class for questions or doing one-on-one conferences outside of class with students can help alleviate this feeling. However, students often cheat and plagiarize because it's easy or they think they won't get caught.
Regardless of what the student intended, it is difficult as teachers to detect plagiarism once a paper has been turned in.