例句：
When will the single currency be operable?
什么时候实行单一货币？
When's food? I'm starving!
饭什么时候好？我快饿死了！
Some people don't know when they're well off.
有些人身在福中不知福。
I didn't use to like him much when we were at school.
以前我们同学时，我并不太喜欢他。
When is the best time to ring New York?
什么时间给纽约打电话最好？