英文中的formal和informal

Formal English : a style of speech or writing that is very correct and suitable for official or important occasions 正規英語‧用於正式場合如 考試、公司文件、政府公文、文學著作等等。
Informal English : English that is used between friends or in a relaxed or unofficial situation, such as normal everyday conversation 非正式英文‧用於日常談話、親朋間書信往來等非正式場合。
The difference probably stems from usage (慣用法) --- the way in which words are used in a language。It is a natural process that has taken place in almost any language, including Chinese 就是我們的母語中文, 也有 formal 與 informal 之分, 例如作文時我們會寫 “召妓” 和 “自作自受”, 平常交談會講 “叫 x” 和 “抵死”, 其實每種語言都有同樣情形出現, 這是語言進化的必然結果。
Examples :
When we talk, we say "Don't mess around with him, he's a nut." But in an essay, especially when writing for an exam, it is much better to use "he is a strange person" or "he is weird". The word 'nut' (怪人) is an informal word, replaceable by 'strange', 'crazy' or 'weird', words that are acceptable in serious occasions.
Another informal word is 'dig' (熱愛、勁喜歡). You can write in your blog "I dig rock & roll music"；but in your resume you should write "I like rock and roll music very much", or "I have a passion for rock and roll music".
It is important that we differentiate between formal and informal words. Besides the possibility of causing embarrassment (尷尬), it may also cost us our marks (in exams) or even our job opportunities (when writing an application letter).
If you have doubts about formal and informal words, consult a decent dictionary. I'm sure you'll find this helpful.

如果需要中文版本，請寄電郵給我。
If you need a Chinese version, please send me an e-mail.

In English, informal languages include slang, ellipsis and idiom, and an example of an idiom is 'it rains dogs and cats' the meaning of this sentence is it rains very heavily. Informal languages also includes unnecessary abbreviation like ok instead of okay, also ‘u’ instead of you, and ‘n’ instead of ‘in’ and ‘and’. Definitely swearing words, but apart from swearing words, it will not include some rude words like ‘idiot’, in formal text we won’t use these words.

While writing formal text, if you need to write a verb to be behind a pronoun (i.e. I, you, they, he, she, it) don’t write short form or ain’t but the original form. For example, He is rather than He’s. Another thing is, when adding not to a verb to be or verb to do, don’t use short form.

In pronunciation, some words ending with ing (for example walking, running), some people will pronounce as walkin and runnin, this is an example of informal too.

In very formal text, there will be archaisms, this means languages which is very old.

The text that without any capital letter and puntuation is also an example of informal, but you can often seen that happens in messegers and forums.

You can use formal language all the time, but if you are talking to someone who is very close to you, informal language seems to be friendlier. The only time that you can use informal language is when you are talking to someone who is very close to you. If you use informal language incorrectly, the person/people you are talking to may feel embarrass, so if you are not sure to use informal or formal, you should use formal.

Example:

Formal
A: It rains very heavily.
B: Yes, maybe, I will not go.

Informal:
A: It rains cats ‘’n’ dogs.
B: Yes, maybe, I won’t go.

FORMAL
正中, 傳统, 有禮貌, 绩業性
學校文章, 工事上文件/書信 (有時侯説話/重要文件)

INFORMAL
簡單, 亲切, 普遍性
亲戚朋友書信, 平是對話, 同事之間的簡單對話 (EMAIL/内部電話 ETC)

Formal=正式,嚴格,保守,傳統的
所以在酒會,公司party,開會,寫信或任何嚴肅的場合都要用Formal嘅詞語.
例:
A: Thank you for coming this evening.
B: It's my pleasure.

注意: 寫信盡量避免用簡化詞語,如It's, I'm, he's, OK等.

Informal=非正規,不拘形式,口語化(保守嘅人可能會視為不正當)
在與朋友,家人,親友對話時,或寫便條時使用,可簡化詞語,不用遵守禮節(粗言穢語就唔得la,也要保持基本禮貌)也可以直接叫人first name
例:
A: Hi Peter, do you want to go swimming today?
B: Sure, I'd love to come.

希望可以解決你嘅難題!