Types of register
There are of course many different registers of a language, and some words are only appropriate in particular contexts. We can simplify these as follows:
- literary / poetic
The most informal registers are those used in informal conversation. Slang may be used, many phrasal verbs and expressions from the Anglo Saxon part of the language. As the language becomes more formal - and written - there is greater tendency to use long sentences, complex verb structures and vocabulary from the Latinate part of the vocabulary.
Informal registers are not "wrong"
There is sometimes a tendency to regard registers which use phrasal verbs, forms with "got" and contractions as somehow "wrong" - but this is not the case. It is simply a question of using the correct register for the situation. Whereas native speakers usually do this quite naturally, teachers will probably need to explicitly make this point to their students. Furthermore, although written work has traditionally been regarded as a more formal register, emails, postcards and text messages can be outstandingly informal.
A particular problem for students whose mother tongue is a Latinate language is the use of cognates from their mother tongue which may make them sound excessively - even absurdly - formal in some situations.