Complex sentence: As a boy, Radványi attended a gymnasium in Budapest on Marko Street where, at 16, he authored a book of poetry, which received a preface from Frigyes Karinthy.
Split into simpler sentences: As a boy, Radványi attended a gymnasium in Budapest on Marko Street. At this same gymnasium, at age 16, he authored a book of poetry, which received a preface from Frigyes Karinthy.