ものを

ものを laments a better outcome that was within reach and did not happen: "if only you had…", "you could have just…". It goes after the plain form, most often after きごう〜ばいい or きごう〜ばよかった, and the second half says what went wrong instead.

When it follows someone else's action, ものを blames them. With きごう〜ば and a past verb it says what would have happened.

Stopping at ものを

The sentence can end on ものを and leave the bad result unsaid. This is its sharpest use: a reproach or bitter regret hanging in the air.

ものを or のに

のに covers the same ground and is what you say in everyday speech. ものを sounds old-fashioned and bookish, and carries more reproach, so you meet it in novels, drama and the occasional pointed remark.