〜ものを
Formation
Meaning
Used to express regret or dissatisfaction, implying that if only a certain action had been taken, a different (usually better) outcome would have occurred.
Note
Expresses strong regret, lamentation, or frustration about a situation that cannot be changed. It implies that a better outcome was possible if a different choice had been made. It is more formal and emotionally heavy than 'noni'. It is frequently used with the hypothetical or potential forms ('could have', 'would have'). In many cases, the final clause is omitted, leaving the regret hanging.
Examples
連絡してくれれば迎えに行ったものを、水臭いじゃないか。
I would have come to pick you up if you'd just contacted me. Why so distant?
もうちょっと頑張れば合格できたものを、本当に惜しいことをしたね。
If you'd tried just a little harder, you could have passed. Such a shame.
もっと早く言えばいいものを、どうして黙っていたの？
If only you'd said something sooner—why did you stay silent?
安ければいいものを、こんなに高いと買えません。
It would be fine if it were cheap, but if it's this expensive, I can't buy it.
静かなものを、わざわざ騒ぎ立てることはない。
It's already quiet, so there's no need to make such a fuss.
便利なものを、あえて使わない理由がわからない。
It's so convenient; I don't see why you'd go out of your way not to use it.