You must use citations when:
When in doubt, cite it!
The Oxford English Dictionary defines Plagiarism as:
The action or practice of plagiarizing; the wrongful appropriation or purloining, and publication as one's own, of the ideas, or the expression of the ideas (literary, artistic, musical, mechanical, etc.) of another.
Try this excellent tool. It is simple to use and will save you time.
NoodleTools is a bibliographic citation and notes management tool. It supports you in creating citations for various source types in the appropriate style, and also helps you keep those citations associated with the notes that you take on the sources you use.