Expresses regret about a situation that did not happen; 'if only', 'it would be good if'.
Verb-casual, past + ば + Verb-casual, past + ものを, い-Adjective + ければ + い-Adjective + ものを, な-Adjective + であれば + な-Adjective + ものを, Noun + であれば + Noun + ものを
The ～ば～ものを grammar point is used to express regret for a lost opportunity or to show that an ideal condition was not met. This grammar point is often translated as 'if only' or 'it would be good if' in English. It can be used with verbs, い-adjectives, な-adjectives, and nouns.