Often in our writing the issue is not one of correct grammar or spelling but rather one of how to best convey our point to the reader. The words we choose and how they go together can affect the reader's perception of our company, the message, and us. People assess our expertise, education, and background based on these criteria. Thus it is important to know your audience enough to gauge the tenor of your writing.
The style or tone chosen should fit the occasion as well as the intended audience. You wouldn't want to use formal language when your intent is to be laid back or irreverent, just as you wouldn't want to use an informal tone when writing a policy manual.
Often one's writing will have a mixture of formal and informal elements. For most writers whose words are intended for a general audience the trick is to strike a balance between a tone of slight formality and being downright stuffy.
In general, formal writing is characterized by the tendency to adhere closely to the rules governing grammatical sentences and to avoid expressions and word use that would be used in more casual missives. However, this does not mean that formal writing is intentionally pretentious (using language that seems intended mainly to impress readers). Situations that would call for formal usage would include journal submissions, official reports, job applications, and articles regarding serious subject matter.
Informal writing incorporates many of the familiar features of spoken English. Measures of informality include the use of colloquial terms, contractions, and the tendency to abbreviate sentences by omitting certain elements.
An important factor to remember is that formal and informal refer to the tone or style of your writing, not to grammatical correctness. Each style should conform to grammatical rules. The two styles are simply used for different occasions.
- Imagine how you'd talk to your friend on the phone.
- Imagine how one would speak on a job interview.
- I won't have that report done today.
- I will not have the report finished today.
- Let's stop for the night.
- Let us adjourn for the evening.
- You eat sushi with chopsticks, not a fork.
- Sushi should be eaten with chopsticks, not with a fork.
sources: The Chicago Manual of Style; Fumblerules by William Safire; The American Heritage Book of English Usage; the Internet