We don't realize how valuable it is until we lose it.
If we had realized how valuable it was, we would never lose it.
I wish we could have realized how valuable it was before we lost it.
いろいろと表現してみました。
We don't realize how valuable it is until we lose it.
失くして初めてその価値に気づく。（失くすまでその価値に気づかない）
If we had realized how valuable it was, we would never lose it.
仮定法を使いました。
その価値が解っていたなら、決して失くさないだろう。
I wish we could have realized how valuable it was before we lost it.
これも仮定法です。
失くす前にその価値がわっかていたならなあ？
日本 
