ものを

ものを is a sentence-ending or mid-sentence particle combination that expresses a strong sense of regret, reproach, or disappointment about a situation that is contrary to what was expected or desired. It often follows a clause that describes what happened or what was thought would happen, and the following clause (usually in the past or a contrastive expectation) carries the speaker’s rueful feeling. It can be used to lament something that did not occur, something one wishes had been different, or a missed opportunity. This construction is common in more formal or literary-like speech, and in spoken Japanese it tends to sound slightly old-fashioned or emphatic.