〜ばいいのに・〜ばよかったのに
I wish; if only; you should have
ば-form / たら + いいのに (now) / よかったのに (past)
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Adding のに to ばいい or たらいい turns advice into a wish: もっと時間があればいいのに, "I wish I had more time". The のに adds "but it isn't so", with a touch of regret or frustration.
For the past, it's ばよかったのに: 言ってくれればよかったのに, "you should have told me". Aimed at someone else, it's a gentle reproach.
たら works the same way: 明日が休みだったらいいのに, "I wish tomorrow were a day off".
The choice between いいのに and よかったのに depends on time: a wish about now or the future takes いいのに; regret about the past takes よかったのに.
Examples
もっと時間があればいいのに。
I wish I had more time.
雨が早くやめばいいのに。
I wish the rain would stop soon.
彼も来たらよかったのに。
It's a shame he didn't come too.
言ってくれればよかったのに。
You should have told me.
明日が休みだったらいいのに。
I wish tomorrow were a day off.