Contents introduction chapter I. Bases of english language and its stylistic layers

Relative Structures:

Use relative structures: The woman thought that it was important to be on time.	Drop certain relative structures: The woman thought it was important to be on time.
Use 'whom' as an object: Whom have they chosen for the position?	Use 'who' as an object: Who have they chosen for the position?
Auxiliary Verb Usage:
Always use the full form of an auxiliary: Have you finished your work?	Sometimes the auxiliary verb is dropped in informal speech. Note: This is often grammatically incorrect, but is certainly common in everyday usage: Finished your work?
Word Choice:
Many words tend to be used in more formal situations. For example, certain verbs tend to be used in formal situations, but have other synonyms (often phrasal verbs) that are used in informal situations. Also, it uses more vocabulary derived from French and Latin (bigger words) It uses more synonyms and doesn't repeat the same words as much. The police investigated the situation.	We looked into the situation
Passive structures:
It uses more indirect language, making it less personal. The student was given a book.	We gave the student a book.

The differences between the casual and formal styles of English may be broadly classified in terms of its vocabulary, word order and punctuation. Nevertheless, there are other aspects as well. In spoken English, the casual style of English which is commonly referred to as the informal English is used but in academic writings, it is a must to use the formal style of English. It is essential for all the English learners to accumulate as much inputs as possible. However, you must be aware that not all the inputs are the same. 

You would find formal English most commonly in textbooks, official documents, business letters, newsletters, reports, articles whereas informal English is commonly used in our day to day conversations. The need for formal English arises when you try to write an official letter or give a presentation or read a book. However, this doesn’t mean that you should neglect informal English as it is equally important to understand and communicate with native speakers in your day life.
Here, let us try to differentiate between formal and informal English in terms of vocabulary. Some major differences are:
1. Formal English are properly edited and used carefully. The person using formal English has ample time at hand to polish the text to achieve near perfection. While writing down quick notes, personal emails or chatting, informal English is commonly used.
2. Formal English is commonly used in giving speech, presentation etc. Whereas, informal English may be referred to as improvised speech as the speaker is speaking without any preparation.
3. In formal English, the sentences are complicated and longer. Whereas in informal English , short and simple sentences are used.
4. The standard correctness of formal English is higher as compared to informal English. That is to say, the sentence that may be considered as correct in informal English may be wrong in formal English.
5. In formal English, use of heavy words, phrases and complex sentences are quite common. Whereas in informal English, the sentences comprises of everyday phrases that is communicative and easy to understand.
Thus, formal and informal languages are equally important for a learner to understand so as to fit in themselves in every kind of situation. Get hold of the different sources from where you can gather all kinds of inputs to be able to clearly classify between formal and informal English. Some input sources for your reference are forum posts, blogs, emails, news articles, movies, TV series, radio conversations, talk shows etc.
We use formal language in situations that are serious or that involve people we don’t know well. Informal language is more commonly used in situations that are more relaxed and involve people we know well.