（よ）うものならIf someone were to dare or attempt...
This pattern gives a serious warning: if someone were to attempt the action, or if the event occurred even once, a damaging or otherwise unwelcome consequence would follow.
How the warning works
The first clause presents an action or event as a dangerous possibility. The following clause then states the punishment, damage, loss, scandal, or other serious result that makes the speaker warn against it. The volitional form often suggests someone attempting the action, but here it does not express an ordinary intention.
この情報を外部に漏らそうものなら、会社全体の信用が失われる。
If anyone were to leak this information outside the company, the entire company would lose its credibility.
AnalyzeThe loss of the company's credibility supplies the serious consequence behind the warning.
Conversational contraction
今、予備電源を切っても大丈夫ですか。
Would it be all right to turn off the backup power now?
Analyze運転中に切ろうもんなら、装置が全部停止する。点検まで待って。
If you were to turn it off while the system is running, all the equipment would stop. Wait until the inspection.
Analyzeもんなら gives the warning a distinctly conversational voice.
Events without a deliberate actor
The form can occasionally describe a nonhuman change rather than a deliberate attempt. The point is still the same: even one occurrence would trigger a dangerous result.
この古い配管は、圧力が少しでも上がろうものなら、破裂するおそれがある。
If the pressure in these old pipes were to rise even slightly, there would be a risk of rupture.
AnalyzeThe pressure has no intention; the volitional-looking form marks its occurrence as a dangerous possibility.
Do not confuse the verb forms
Volitional form + ものなら warns of an adverse consequence. Potential form + ものなら instead means roughly “if it were possible” and often introduces a wish. Ordinary hypothetical forms such as としたら do not inherently carry a warning, while ようとも says that the result remains unchanged even if something happens.
機密を漏らそうものなら、厳しい処分を受ける。
If anyone were to leak confidential information, they would face severe disciplinary action.
AnalyzeVolitional form: a warning about the result of attempting the action.
もう一度会えるものなら、直接謝りたい。
If I could meet them once more, I would like to apologize in person.
AnalyzePotential form: a remote possibility connected to something the speaker wishes to do.
More examples
この契約内容を無断で変更しようものなら、損害賠償を請求される可能性がある。
If you were to alter the terms of this contract without permission, you could face a claim for damages.
Analyze一歩でも足を踏み外そうものなら、谷底まで滑落しかねない。
If you were to lose your footing by even one step, you could fall all the way to the bottom of the ravine.
Analyze患者の情報をインターネット上に投稿しようものなら、病院の信頼は一瞬で失われる。
If anyone were to post patient information online, the hospital's credibility would be lost in an instant.
Analyze彼が会議でその事実を口にしようものなら、社内は大騒ぎになる。
If he were to mention that fact at the meeting, the whole company would be thrown into an uproar.
Analyzeこの支柱が一本でも折れようものなら、屋根全体が崩れる危険がある。
If even one of these supports were to break, the entire roof could collapse.
Analyze証拠を隠そうものなら、かえって疑いが深まる。
If you were to try to hide the evidence, it would only deepen the suspicion.
Analyze