The text I posted below was corrected yesterday. I must say I was a bit disappointed. The native speaker luckily gave me some good advice: I need to focus on prepositions and the use of definite and indefinite determiners. I attribute the second big mistake to distraction, though, as I was tired and I normally do not make many mistakes with determiners. I do agree that I need to work on prepositions. Because I have no desire to wait until I am halfway through Harry Potter before I know all of them, as the challenge will end in less than three months, I made a list of (71) prepositions which I am going to memorise. This is not my favourite method, but if it helps, it’s good. This way, I can accelerate the learning process a bit.