The English language has borrowed intensively from other languages. The result is an intense vocabulary of some million words, many with similar meanings. Roget's Thesaurus, for example, lists almost 100 synonyms for insane and over 150 synonyms for destroy . From this abundance, writers choose the words that best fit intended meaning and individual styles.
Each time you write, you should decide whether to use a formal or an informal voice. The decision depends on your purpose and your audience. A formal voice is appropriate for business correspondence, reports, research papers, and articles in scholarly journals – documents in which writers distance themselves personally from the readers. While an informal voice is appropriate for purposes such as humorous writing, advertising, and articles in popular magazines – material in which writers try to establish a personal relationship with readers. Here, you should use a formal style to establish a polite, professional relationship with a reader and an informal style to establish a friendly, conversational relationship.
The degree of formality or informality is established in large part by vocabulary. Words derived from Anglo-Saxon (Old English) seem more informal and conversational than words derived or borrowed from other languages. For example, the Anglo-Saxon derivatives lucky, get, buy and crazy seem less formal than their synonyms derived from Greek and Latin: fortunate, obtain, purchase, and demented . Likewise, the English words therefore and masterpiece are less formal than theirs Latin counterparts ergo and magnum opus .
Clipped forms are more informal than full forms. For example, pro, ad and deli are more formal than professional, advertisement, and delicatessen . Likewise, contractions (can not, is not, it's) are more informal than uncontracted forms (can not, is not, it is).
First person (I, we) and second person (you) are less formal than third (one, the writer, the student) . If you are writing about yourself, I certainly seems more natural than one or this writer . If you are addressing a reader personally, you seems natural. Avoid, however, using you to mean people in general.
Slang is informal – sometimes, very informal – and its appearance in formal documents can reduce them to the absurd. Imagine, for example, reading something like this in a college bulletin: "Students with wheels should boogie on over to the security office and get a decal." However, a carefully chosen slang expression can make prose more interesting, viable or efficient. "Razzmatazz" is more interesting than "a flashy display." "Bug a telephone" is more viable than "equip a telephone with a microphone." "Computer nerd" is definitely more efficient than "a person who promises the social amenities in an obsession for computers." Remember that an abundance of slang will make prose seem silly. Furthermore, the meanings of slang expressions are frequently unstable – changing unpredictably from time to time and audience to audience.
Choosing a formal or informal voice is often arbitrary; in many circumstances, readers will accept either. But whichever you choose, you should maintain it consistently through a composition. Notice how the voice in the following passage appears to shift from formal to informal and back to formal. As a result, the reader gets mixed signals about the writer's attitude.