ようものなら: consequences if someone were to act
Use ものなら after a volitional form to present a risky hypothesis. Distinguish an attempted action from actual completion and from wishes using a potential form.
Warning of serious consequences
秘密を話そうものなら、信頼を失います。 warns that revealing the secret would cost someone's trust. Although 話そう is volitional, the speaker is not promising to disclose the secret here.
The expression emphasizes a serious or unwelcome result of the hypothetical action. Unlike an everyday sequence such as ご飯を食べたら出かけます, its central stance is a warning of risk.
The preceding form and whether anything happened
勝手に入ろうものなら、すぐ注意されます。 means that attempting to enter without permission would bring an immediate reprimand. It does not report entry or a reprimand that already happened.
Form the volitional for the particular verb, as in 書こうものなら, 見ようものなら, and しようものなら. Do not simply append よう to every dictionary form.
Comparison with potential-form ものなら
入れるものなら入りたいです。 wishes to enter if possible. Compared with 入ろうものなら, the difference is a wish about feasibility versus a warning about taking action.
An ending such as 警報が鳴るでしょう makes the warning's result a prediction. Despite the forceful expression, preserve the following predicate's time and uncertainty rather than adding an actual occurrence.
Quick check: Does 遅刻しようものなら、厳しく注意されるでしょう。 report being late and already receiving a reprimand?
Show the answer and why
No. It warns and predicts that being late would lead to a stern reprimand. Neither the lateness nor the reprimand is reported as completed.