Student's guide to writing college papers, Fourth edition - Kate L. Turabian 2010

Don't plead ignorance, misunderstanding, or innocent intentions
Preventing plagiarism
Writing your paper

To be sure, what looks like plagiarism is often just honest ignorance of how to use and cite sources. In those cases, students defend themselves by claiming they didn't intend to mislead. The problem is, we read words, not minds. So think of plagiarism not as an act you intend but as one that others perceive. Avoid any sign that might give your readers a reason to suspect you of it. Whenever you put your name on a paper, you implicitly promise that you wrote every word that you don't clearly and specifically attribute to someone else.

Here is how to think about this: If someone read your paper immediately after reading your source written by Johnson. Would she think, This sounds just like Johnson or I remember these words or This idea must have come from Johnson. If so, you must cite Johnson and set off any sequence of his exact words in quotation marks or a block quotation.