If only one could: 〜ものなら

〜ものなら, after a verb in the potential, considers something almost impossible: “if only it were possible…”. What follows expresses a wish (〜たい, 〜てほしい) or, in a defiant tone, an order (〜てみろ: “go on, try it if you can!”).

• できるものなら、もう一度学生に戻りたい。できるものなら、もういちどがくせいにもどりたい。If only it were possible, I'd love to be a student again.
• 代われるものなら、代わってあげたい。かわれるものなら、かわってあげたい。If I could take your place, I gladly would.
• やれるものなら、やってみろ。やれるものなら、やってみろ。Go on, try it if you think you can!

もの + なら (“if”) after a potential creates a hypothesis you know is almost unachievable: “if it were possible, though it isn't really”. It's not a realistic condition: it's a sigh or a dare.

Two colours: the wish (できるものなら〜たい) and the provocation (〜れるものなら〜てみろ: “go on, I'm waiting”).

• やり直せるものなら、あの日に戻ってやり直したい。やりなおせるものなら、あのひにもどってやりなおしたい。If I could do it all over, I'd go back to that day and start again.

The verb is almost always in the potential (できる, 行ける, 戻れる). The same verb is often repeated in the second part.

• 帰れるものなら、今すぐ国に帰りたい。かえれるものなら、いますぐくににかえりたい。If only I could, I'd go back to my home country right now.
• 行けるものなら、行ってみろ。いけるものなら、いってみろ。Go on then, go if you can!

With the volitional form, 〜（よ）うものなら takes on a quite different meaning: “if you ever do that, watch out!” (遅刻しようものなら、ひどく叱られる). It's an N1 point: just remember not to confuse it with potential + ものなら.

• あの先生の授業に遅刻しようものなら、ひどく叱られる。あのせんせいのじゅぎょうにちこくしようものなら、ひどくしかられる。If you ever turn up late to that teacher's class, you'll get a real telling-off. (N1)

たら and なら (N4) set up a realistic hypothesis: 時間があったら行く. ものなら assumes it's almost impossible. Saying “時間があるものなら” about an ordinary appointment would be strange.

• 時間があったら、手伝います。じかんがあったら、てつだいます。If I have time, I'll help you. (realistic hypothesis)