The English language is definitely not the hardest language but it surely is one of the hardest languages to learn. A native speaker might easily pronounce a word that they have never but for a person who is Learning English as a second language; it might be a difficult task. However, when it comes to differentiating between formal and informal English, it might not be that difficult as there are clear differences between the two. It is important to know the differences so as to be able to use the English language in an appropriate manner whenever required.
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.