Formal English is the one used in schools, when you write letters....
Informal English is the one spoken in everyday life. Native speakers, American children speak this kind of English. Informal English is also called "Casual English" or "Everyday English".
For you as an English student: which English should you learn? Everyday English or Formal English?
Well the answer is very simple: the best decision is to learn both!
Many English students confuse this question ( which English should I learn: formal English or informal English?) with: which English should I start to learn first?
For the first question, the answer is simple: learn them both.
But for the second question, there are more details:
Well, it is a little bit difficult to decide whether you should start with casual English or formal English. Some English teachers say: "start with formal English, so you can get a job, writer administrative letters..." and some other English teachers say: "no, start with everyday English, so you can speak like an American or like native speaker, you can understand native speakers..."
For me, as an English teacher, I won't tell you to start with formal English or informal English, but I'm going to tell you, according to your case, which English should you start with:
What is your goal when you're learning English? why are you learning English? are you learning English for business? for your studies? to get the TOEFL? to speak English?
As mentioned, it depends only on you. I'm going to explain some details for you and I think you will immediately decide yourself which English you should learn:
If your goal is to pass an English test, for example to get a job, I mean a formal test of course, then you should learn formal English and once you get a good level, you start to learn casual English.
If your main goal is to speak with native speakers then you should start with everyday English and once you speak like native speakers, you start to learn grammar and formal English.
OK, and if you don't have a goal, or you don't know what to do?
If you have time (you're just learning English to have fun) and you don't care if you learn formal English first or casual English, I recommend that you begin with "everyday English". I chose this for two reasons:
1-When an American child starts to learn English, he first learns everyday English, and once he can handle it, he starts to learn grammar and formal English. And remember our rule: you should learn English like children.
2-Even in business nowadays, they don't use formal English. Formal English is going to be useless step by step. Try to listen to English radios or to the News and you will see that the English they are using is not formal; they're using casual English!
If you can't know which English they are using, you probably don't know what Formal English and Informal English really are! That why in the next article I'm going to talk about the differences between Formal English and casual English, with examples... You will finally know which English someone is using when he speaks... I think you'll love everyday English rather than formal English. I hope you won't miss the next article.