In a research paper, you will draw on the work of other writers, and you must document their contributions by citing your sources. Sources are cited for two reasons:

Borrowing another writer’s language, sentence structures, or ideas without proper acknowledgment is a form of dishonesty known as plagiarism.

The only exception is common knowledge—information that your readers may know or could easily locate in any number of reference sources. For example, a quick search would tell you that Joel Coen directed Fargo in 1996 and that Emily Dickinson published only a handful of her many poems during her lifetime. As a rule, when you have seen information repeatedly in your reading, you don’t need to cite it.