Formal and informal English
by Tomasz P. Szynalski
The most important piece of advice for an English learner is to get lots of input. But not all input is the same. There are, roughly speaking, two basic types of English input: formal and informal.
Formal English is used in "serious" texts and situations — for example, in official documents, books, news reports, articles, business letters or official speeches. Informal English is used in everyday conversations and in personal letters.
Here is an example of formal English that you might come across in a book:
As the price of five dollars was reasonable, I decided to make the purchase without further thought.
The same thought would be expressed quite differently in informal English. Here's an actual example that I heard from a young American:
It was, like, five bucks, so I was like "okay".
You need to know formal English because you want to be able to read a book, give a business presentation or write an official letter. You also need informal English because you want to be able to understand and communicate with English speakers in everyday situations.