Informal language is casual. It is used when communicating with friends or family either in writing or in conversation. It is used when writing personal emails, and text messages. The tone is more personal than formal language. colloquialisms are very common in informal language. These are terms that are used differently depending on where the speaker may be from such as the words “soda”, “pop”, “soft drink”, and “Coke." Contractions and first person pronouns are also considered informal. Idioms and figurative language work well in informal writing.
Formal language is used when writing for professional or academic purposes. This is a very serious voice. Usually uses third person, unless first person is necessary in a formal letter.
EXAMPLES
Informal: The balloon was blown up for the experiment.
Formal: The balloon was inflated for the experiment.
Informal: It was raining cats and dogs.
Formal: It was raining very heavily
Here's a list of some of the main differences between
informal and formal writing: http://www.word-mart.com/html/formal_and_informal_writing.html
Informal: May use colloquial words/expressions (kids, guy, awesome, a lot, etc.).
Formal: Avoid using colloquial words/expressions (substitute with children, man/boy, wonderful, many, etc.)
Informal: May use contractions (can’t, won’t, shouldn’t, etc.).
Formal: Avoid contractions (write out full words – cannot, will not, should not, etc.).
Informal: May use first, second, or third person.
Formal: Write in third person (except in business letters where first person may be used).
Informal: May use clichés (loads of, conspicuous by absence, etc.)
Formal: Avoid clichés (use many, was absent, etc.)
Informal: May address readers using second person pronouns (you, your, etc)
Formal: Avoid addressing readers using second person pronouns (use one, one’s, the reader, the reader’s, etc.)
Informal: May use abbreviated words (photo, TV, etc)
Formal: Avoid using abbreviated words (use full versions – like photograph, television, etc.)
Informal: May use imperative voice (e.g. Remember….)
Formal: Avoid imperative voice (use Please refer to.….)
Informal: May use active voice (e.g. We have noticed that…..)
Formal: Use passive voice (e.g. It has been noticed that….)
Informal: May use short and simple sentences.
Formal: Longer and more complex sentences are preferred (short simple sentences reflects poorly on the writer)
Informal: Difficulty of subject may be acknowledged and empathy shown to the reader.
Formal: State your points confidently and offer your argument firm support.