"总有一天" 的英语是 "someday"。它来源于古英语中的 "sōme dæg"，其中 "sōme" 是 "某个" 的意思，而 "dæg" 是 "天" 的意思。例句如下：

I know you're feeling down now, but someday you'll be happy again.

Someday I'll win the lottery and buy a big house on the beach.

We may never meet again, but I'll always remember you, and someday we'll be reunited in heaven.