1, The definition of informal words

1,1 Standard informal words

1,2 Substandard informal words

2, The usage of informal words

3, How to avoid using informal words in academical writing

1, The definition of informal words

1.1 Standard informal words

The informal words conclude two parts which are standard informal words and substandard informal words. Standard informal words are also called colloquialism which are always used in our daily life.

1.2 Substandard informal words

Substandard informal words conclude four parts which are slang, vulgarism, professionalism and argot. Slang occurring chiefly in casual and playful speech, made up typically of short-lived coinages and figures of speech that are deliberately used in place of standard terms for added raciness, humor, irreverence, or other effect. Vulgarism is a word or expression originated or used chiefly by illiterate persons, the four-letter words are of this kind. Professionalism are used by the people having the same job. These words are always showing their joking, such as the nurses call the hypodermic syringe as hypo, the soldiers call bomb as buster. They are also called black words, and they are used between the criminals which can’t be understood by us now, such as can-opener, dip, spear.
2, The usage of informal words
Informal words are always used in the oral English in our daily life and are not suitable for the formal occasions. With the help of the various gestures, expressions, postures, and the special situations, the words in the conversations during our daily life don’t have to be as precise as writing or speeches. Several simple words are used very frequently, such as thin, place ,business, do, these words always have a lot of meanings.
A large amount of verb phrases are very active in our talking, such as make up, make down, make away, they also have a lot of different meanings. Some verb phrases can be changed into none which is an important part in the oral English, such as make-up, let down, carry-on.
A lot of back-formation words and shortening words are standard informal words, such as baby-sit, math. Some adjectives and adverbs which have already lose their own meanings, but play the functions of emphasizing, such as awfully, wonderful, lovely.
Some slang are used for a very short time, such as hooch, baloney, sheik, tine lizzie, which are on longer used now. Some slang are still used today, some are even used as colloquialism or standard informal words today, such as bet, bone, trip, fun, cold war, beatnik. Some slang are very humorous, which can make the communication more interesting, such as back-door.
Vulgarism are always ungrammatical ,such as knowed, drownded, would of (would), seen (saw). These words make the talking vivid.

3, How to avoid using informal words in academical writing

Written academic English will not normally contain the following: contractions, hesitation fillers, personal pronouns, imprecise words, abbreviated form for example:
it didn’t
it did not
they have
hesitation fillers

personal pronouns
I think this is an effective plan.
You put the chemicals in the
test tube.
We used two different
methods research.
This could be an effective plan.
The chemicals are put in the 
test tube.
There were two different
methods of research
imprecise words
lots of
excellent, pleasant
reasons, problems
such as
abbreviated form
it is
they are
for example
that is
and so on
There are several steps that can help us writing academical article properly. It is helpful to understand the differences between formal and informal English words as the first step. They differ in word choice, word usage, and grammatical structures. Informal writing may sound more like conversation while formal writing may be more polished. for example:

This guy, Hewson, right, he says that people that speak English, like native speakers, don’t think about
when to use words like ‘the’, they just do it.

Hewson (1972, p. 132) has called the English article system a ‘psychomechanism’, through which native
speakers use articles correctly but unconsciously
Use appropriate punctuation. For example(as below), American English employs a colon in a formal letter as in “Dear John:” but British English employs a comma.1 Limit parentheses, exclamation points, and dashes (prefer colons) in formal writing. Avoid the ampersand (&); write out the word “and.” Punctuate your writing as you go along to reduce your risk of leaving out punctuation.
  • An informal letter:
  • John,
  • I’m looking for a job, and I’ve heard through the grapevine that you need a workhorse for your shop. Well, I’m the man of the hour, as I’ve got a lot to offer. I’m pretty hard-working, and I’m really good about being on time. I’m also used to working by myself. Anyway, tell me whether you want to get together for an interview, okay?
  • -Informal Joe
  • A formal, professional letter:
  • Dear John:
  • I understand that you are looking for a strong worker to assist you in your shop. I would appreciate consideration because I am diligent, punctual, and accustomed to working with minimal supervision. Please contact me if you are interested in arranging an interview. I thank you for your time.
  • Respectfully,
  • -Professional Joe
Avoid common colloquial words and expressions (colloquialisms). This includes slang such as "cool," "dude," and "humongous." Two good phrases to delete are "you know" and “you might be thinking.” You do not have the power to know your readers’ thoughts while they read your paper. Another empty sentence is “Think about it.” Assume that your readers are already thinking about what they are reading, and state your point more clearly. The adverb “pretty,” meaning “relatively," "fairly," or "quite,” is unacceptable in all formal writing and is often unnecessary.
Try to avoid the first and second person. For example:
You should drink 8 glasses of water everyday.(informal)
One should drink 8 glasses of water everyday.(formal)
Always include the relative pronoun.2 In formal English, it is always a must to use "whom" , "which"or "who" , for example:
This is the article I found in the internet. (informal)
This is the article which I found in the internet. (formal)
Never start a sentence with a coordinating conjunction. For example: never use "but", "so", “and” as a starting word of a sentence. One can use transitional adverbs such as “additionally” (or “moreover”), “nevertheless” (or “however”), “therefore” (or “thus”), and “alternatively” (or “instead” or “otherwise”) to start a sentence.
Avoid stage directions. Do not commence a letter by telling the readers what is the topic of this article.
Avoid vague words. Vague words are less formal and are open to interpretation; they do not express the ideas as well as more precise words would. "A few" or "enough" can often be replaced by something more precise.
Avoid cliches .3
By following the steps showed above, one can write a real academical paper.

(Miller, JL 2005, ‘Most of ESL students have trouble with the articles’, International Education Journal, ERC2004 Special Issue,
vol., 5, no. 5, pp. 80-88)
1, Formal Email Format Handout.
2, Formal vs. Informal. Gnosis Learning.
3, Richard Lederer, Richard Dowis, and Jim McLean. Sleeping Dogs Don't Lay: Practical Advice for the Grammatically Challenged. Macmillan, 2001. Page 100.