We never know the worth of till the well is dry.

例句：

1. 1. I willnot leave until you have finished your work.
   我等你工作完成才会离开。
2. 2. The childrenplayed in the park until it got dark.
   孩子们在公园玩到天黑才回家。
3. 3. She waited for her friend until hearrived.
   她等她的朋友直到他到达。