We searched in vain for the missing earring. [=we searched for the missing earring but did not find it]我们寻找丢失的耳环，不过是白费力气。

He tried in vain to get the baby to sleep.他努力想哄宝宝入睡，却是徒劳一场。

Her suffering will not be in vain. [=something good will happen because of her suffering]她不会白白受苦的。