Before leaving the room, he turned off the radio.
这句话是新概念第二册第20课的例句。这这一课语法知识点是动名词。动名词不是不能做状语么,为啥这句话中动名词做了状语?岂不是矛盾了?
Before leaving 是“介词+动名词”结构,属于介词短语,可以作后置定语和状语,在网友提供的句中作时间状语。又如:
Before sending a letter, you must put a stamp on the envelope. 寄信前,必须在信封上贴邮票。(赵俊英:《现代英语语法》p.454)