"When do I cite a source?"
Whenever you include a word, phrase, or idea from a source, it needs to be cited. That source can include a web page, classroom lecture, an interview with your Uncle Pete, a quote or summary from a book, magazine, etc. And note the "a" word: singular. Even a single word from someone else, when included in your own essay, needs to be set off with quotation marks and then cited.
"When do I use MLA?"
In general, classes in the Humanities (English, History, Art) use MLA style citation. You should always check with your professor and ask which style they would prefer for a particular assignment.
"How do I cite using MLA?"
The number two is important to remember when using MLA citation because it consists of two parts: an in-text citation (which includes the author and usually the page number) and a works cited entry (included on separate "Works Cited" page at end of essay), as shown in the examples below:
Sample In-Text Citation (the part that goes in your essay)
Work for a living? Feeling a bit poor lately? The link between the two is not difficult to explain when you examine recent pay raises. In 1997, the average worker received a raise of 3 percent. Her CEO? Many earned a 21 percent raise (Cooper 339).
Note that the author's name and page number is enclosed in parenthesis (no p. or page is needed) and that the period goes to the right of the parenthesis.
Sample Works Cited Entry (the part that goes on a "Works Cited" separate page at the end) for in-text citation above:
Cooper, Mary H. "Income Inequality." CQ Researcher 17 April 1997: 337-360.
Together, these two parts let the reader know who wrote your article, where they can find it, and approximately how long the article is.
This two part citation method works like a code. In general, the reader looks for the author’s name and page number in the in-text citation (or the first word of the title if there is no author), and then goes to the works cited entry for additional information. Your job is to supply the correct parts of the code in the correct order.
You have to be sure that the name/word you include in your in-text citation will match the first word of one of your works cited entries. Thus, the word “Cooper” from the in-text example above (in the parenthesis) matches the word “Cooper” in the sample works cited entry.
Readers would see the citation (Cooper 339) and instantly know that the information before the citation is from a writer named “Cooper.” If they wanted to check your source, they would turn to the Works Cited page and scan the first word of the alphabetically arranged list of Works Cited entries until the word “Cooper” appeared.