It's all about register...

register. R E G I S T E R. /ˈredʒɪstə(r)/
Linguistics a variety of a language or a level of usage, as determined by degree of formality and choice of vocabulary, pronunciation, and syntax, according to the communicative purpose, social context, and standing of the user. (Oxford Dictionaries)

When you start to learn a language you are glad when you remember words, phrases and sentences. You are glad when you are able to talk, understand and answer. By improving your language skills and by reaching a higher level step by step, you realise that this is simply not enough. Someone, in my case my English teacher, will tell you that not every word is appropriate in every situation and not every phrase fits every conversation. You start to realise that the whole language learning thing is about much more than just grammar or vocabulary. You will stumble across a word called "register" and you will soon realise that this word is more important than you might at first want it to be.

Register is about using the appropriate standard of English in different situations and for different purposes. It might for example not be the best idea to greet your boss at work or your teachers with "hey guys" because this is simply not the appropriate register. There are some major differences between talking to your close friends, acquaintances or your boss. Therefore it is necessary to look up in which situation and register you use a word when you learn new vocabulary. 

It's not as difficult as it sounds because once you understand that it's the same like in German, it gets much easier. Just imagine writing an e-mail to a professor. You wouldn't start your message with "hey" and go on with whatever you want to say, would you? Maybe if it's a teacher you don't know so well you would write something like "Dear Mr. XY..." or just "Dear ..." and the name of the person if you know them better. It always depends, but I'm sure that you will make a much better impression on the person you are writing to if you are polite and respectful. 

Of course this is just the most basic information when register is concerned but it's a start. As your language skills improve it becomes more complicated and you have to be careful because for example it's not appropriate to start a sentence with"but" or "so" in a text of higher register. On the contrary it might look strange if you use the word "hence" in a conversation or a text of lower register. 

As you can hopefully see using the appropriate register is essential when you want to improve your language skills because it's all about register...