Tā / chū / mén / qián / cái / fā / xiàn / Tiān / ān / mén / kāi / le / gǎn / jǐn / huí / fáng / jiān / huàn / kù / zi

他出门前才发现天安门开了，赶紧回房间换裤子。

He only realized before going out that his fly was open, so he hurried back to change his pants.