adj.徒劳的,不成功的;无望的;自负的,虚荣的;
例句:
I don't like a vain woman.
我讨厌爱慕虚荣的女人.
Their blood was not shed in vain.
他们的血没有白流.
All our efforts were in vain.
我们的所有努力都付诸东流了。