You never know means anything could happen, just in case. It encourages someone to try, even when the odds are low: Send the application anyway; you never know, they might have another opening. It also explains a precaution: I packed an extra pen — you never know. You here means people in general, not the listener.
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You never know; something might happen. / statement — you never know.
- Send the application anyway; you never know, they might have another opening.
- I packed an extra pen because you never know when one will stop working.
- You never know; the weather might improve before the concert.
What changed
First versionI might ask whether there are spare tickets; you don't know.
RevisionI might ask whether there are spare tickets; you never know.
You don't know looks like a simple way to say nobody can be sure. But it tells the listener that they personally are unaware. The fixed phrase is you never know: 'nobody knows, anything can happen'.
Who knows? is close, but often comes with a shrug: 'no idea, really'. You never know usually encourages: there's a chance, so try. I don't know simply says you have no answer.
Your turn
Encourage someone to try despite an uncertain outcome: Apply anyway; you ___ know.
Show the answer
never — Encouragement uses the fixed phrase you never know. You don't know would sound like 'you're unaware', a criticism rather than support.
A friend isn't sure about going to an interview at their dream company. Encourage them in one sentence with you never know.
Go to the interview anyway — you never know, they might love you.
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