before + V-ing …

结构︰

before + 动名词 …

说明︰

此句型意为“在…之前”。before 后面动作的执行者，应该与<主要子句>中的动作执行者一致。

Upon reaching an appropriate age, children are encouraged, but not forced, to leave the nest.
一达到适当的年龄，孩子们就被鼓励，而不是被强迫，“离开老窝”。
On entering the classroom, I found a book lying on the floor.
一走进教室，我就发现一本书丢在地上。
Upon receiving your letter, I was as happy as could be.
一收到你的信，我非常高兴。