The rules are a check, to be used as an afterthought, in revising. They can prevent you from making silly mistakes, but they cannot make you a good writer. To do your actual writing from the rules will in most cases be a detriment to creativity. Every part of the written work that you hand in should be your own, unless credit is given by way of a specific citation. Please note that the important issue here is taking someone else's ideas .
Plagiarism is literary theft.
When you copy the words of another, be sure to put those words inside quotation marks and to acknowledge the source with a footnote. This is a healthy activity, and I encourage the practice. The interpretation of the piece of literature, the wording of your analysis, and the organization of your arguments should be solely your own. When in doubt, cite.