Formal or informal
People often speak of language as being correct or incorrect. It would be more accurate to refer to particular language structures as being formal or informal, appropriate or inappropriate for a specific context. In everyday conversation we do not have to follow the rules of grammar as carefully as we would in a formal address or a business letter. If we adhere too closely to formal rules of grammar in an informal situation, we can come across as being stuffy and unnatural. It is like wearing a tuxedo or a formal gown to an ordinary business meeting.
Formal English follow rules of grammar very strictly. Sentences tend to be longer and more complex. The vocabulary tends to be elevated, using big words and avoiding slang or vernacular. It avoids split infinitives and prepositions at the end of sentences .
Here are several structures where it is can be helpful to know the level of usage, formal or informal.
Take a quiz on formal English