I was hungry, so I could not concentrate on my homework. → I was hungry, which prevented me from concentrating on my homework. → I was hungry, preventing me from concentrating on my home work. 最後の書き換えパターンでは,"I was hungry"が分詞構文の意味上の主語なのだと思いますが,分詞構文の意味上の主語として普通には主文の主語であるIに読めてしま