sentence: I have personal experience that dates back to 1987 . In that year , I started a pen friendship with a German girl called Claudia who was 17 years old then .
modified: I have a personal experiment backs to 1987 , in this year I had started a pen friendship with a German girl called Claudia who was 17 years old then .
transformation: DET, MORPH, OTHER, PUNCT, DET, PUNCT, VERB:TENSE