a lost cause

• 徒劳无用的事；注定要失败的事

a lost cause的用法和样例：

例句

1. The battle against the huge empire was a lost cause.
   与庞大的帝国作战是注定要吃败仗的。
2. Trying to help him to improve his pronunciation is a lost cause.
   想帮他纠正发音，那肯定没什么用。