Correção de texto falando sobre a memória

Our memory is extremely powerful. We can memorize many things, for example: special or commemoratives dates, math formulas, we can make calculations, memorize speeches, among other amazing things. However for this must train it becouse a trained memory becomes our best friend, but the question is: How to train our memory? It's simple. Every subject that we are studying must be put into practice so that our minds can absorb as much learning. The memory is a great ally but if not trained, it becomes our worst enemy. As in this case:
Mary: John, Did you go to the doctor yesterday?
John: Oh! No. I didn't remember.
Mary: Have you studied for the exam?
John: What exam?
Mary: Oh My God! Tomorrow you will have an english exam.
John: I forgot!
Mary: John, you have a bad memory.