〜ものなら
if (I) could (but it's unlikely)
Potential verb + ものなら
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ものなら after a potential verb expresses a wish for something unlikely or impossible: できるものなら、もう一度学生に戻りたい, "if only I could, I'd go back to being a student".
The second half is a wish (たい, てみたい) or a request. The speaker suspects it can't really happen, so there's a wistful, longing tone.
As a challenge, it's provocative: やれるものなら、やってみろ, "go on, try it if you think you can". The speaker doubts the other person can.
Don't confuse it with ものの ("although") or もので ("because"). The volitional version, ようものなら, is the next point and has a very different meaning.
Examples
できるものなら、もう一度学生に戻りたい。
If only I could, I'd go back to being a student.
行けるものなら、月に行ってみたい。
If it were possible, I'd love to go to the moon.
やれるものなら、やってみろ。
Go on, try it if you think you can.
戻れるものなら、あの日に戻りたい。
If I could go back, I'd return to that day.
休めるものなら、一週間ぐらい休みたい。
If I could take time off, I'd take about a week.