ENGLISH EXPRESSION: OFF THE TOP OF MY HEAD
There are hundreds of English expressions and considering that there are so many it is quite reasonable to assume that is impossible to remember all of them and even more difficult to use them in every day situations.
The fact that most of you live in Spain makes this task even more difficult because you do not have the English language surrounding you so that you can learn these types of expressions almost organically. It is important to note that in most contexts, especially when you are speaking to other people who are not native speakers, you never hear these expressions because they don't know them either!
So, off the top of my head, I can't really think about any reasons why you would want to learn them, other than if you are speaking to native speakers all the time and even then the most important element is that you able to understand them rather than really use them yourself.
By the way (por cierto), do you know what "off the top of my head" means?
Soy el director de Daniel Smith + Partners. Tengo más de 15 años de experiencia enseñando inglés a españoles.