Here are some examples of how it can be used:
* "They searched in vain for the missing child." (They searched, but didn't find the child.)
* "I tried to convince her, but it was all in vain." (My efforts to convince her were useless.)
* "He pleaded for mercy, but his pleas were in vain." (He pleaded, but it didn't change the outcome.)
Essentially, "in vain" describes an action or effort that is ultimately unsuccessful or pointless.