sentence: The first reason is that I have already studied there three years ago and it was a wonderful experience . I met very nice people from all over the world . I improved my English level thanks to very skilled teachers and I really enjoyed my stay in Oxford .
modified: The first reason is because I already have studied there three years ago and it was a wonderful experience , I met very nice people from all over the world , I improved my English level thanks to very skilled teachers and I really enjoy my stay in Oxford .
transformation: PREP, WO, PUNCT, PUNCT, VERB:TENSE