First the easy part: 친구는 창문을 열어놓았다 = My friend left the window open. This sentence is fine as is, and people will say it this way in most cases. As for "The window was left open by my friend", 친구로 창문이 열려 있었다 is wrong. We never say 로 after a person to mean "by someone". 로 is only used for things/situations. People would use one of these sentences: - 창문은 열어 놓은 채로 있었다 / ...열려진 채로 있었다 (often used, with no "by my friend") - 창문은 친구가 열어 놓은 채로 있었다 (perhaps the most natural, including "by ...") - 창문은 친구에 의해(서) 열려진 채로 있었다 (not as natural - may be heard in police work)