you never know （非正式）很难说，不可预知，很难预料

【例句】
There might be an even bigger one — I doubt it, but you never know.
可能还有个比这还要大的——我觉得有点悬，但也很难说。

Hang in there and you never know what you might achieve.
坚持下去，你永远都无法预料会有什么样的收获。