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I encountered the following multiple-choice question on page 71 (question 5 in
section "1~6") of my JLPT N2 grammar textbook 「新完全マスター文法 日本語能力試験Ｎ２」:

> 大きい地震が（ ）、この家はたちまち壊れてしまうだろう。
>
> ａ 起こらないことには ｂ 起ころうものなら ｃ 起こるとなったら

I chose 「ｃ 起こるとなったら」, but the answer given was 「ｂ 起ころうものなら」. I understand why
「ｂ 起ころうものなら」 makes sense, since this is a clear case of 「もし～たら、大変なことになる」,
which is the definition of ～（よ）うものなら given by my textbook. However, I am not
sure why 「ｃ 起こるとなったら」 is wrong.

My textbook explains ～となったら as

> ### ～としたら・～とすれば・～とすると・～となったら・～となれば・～となると
>
> ～と仮定した場合・～ということになった場合、そうする・そうなる。
>
>
> [...]「～となったら・～となれば・～となると」は、実現する可能性があることにつき、それが実現した場合を考えている。[...]後には話者の判断などを表す文が来る。

Under this definition, 「ｃ 起こるとなったら」 seems to make sense to me:

> (c)（？）大きい地震が起こるとなったら、この家はたちまち壊れてしまうだろう。
>
> This house would probably collapse instantly if a big earthquake were to
> happen.

「大きい地震が起こる」 is a condition that may possibly be fulfilled (実現する可能性があること), and
「この家はたちまち壊れてしまうだろう」 is the speaker's judgment, so the sentence seems to fit
the above explanation. Why is the sentence wrong, and what am I missing here?
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2022-08-19T14:42:27.793
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2022-08-19T21:53:40.037
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question
grammar usage conditionals jlpt
～（よ）うものなら vs ～となったら
If the sentence with option c is read or heard without care, few native
speaker would feel awkward. If careful, majority would not choose c (Some
would, though).

Most probable reason for awkwardness I can think of is that "となったら" is usually
followed by something contrary to the foreseeable/expected future. For
example:

  * 奴らが乗り込んでくるとなったら戦うぜ（戦わないつもりだったが）。
  * 台風が来るとなったら旅行は中止するしかない（どんなに行きたくても）。

In your case, the house must be a rat-trap (because it is presumed to collapse
たちまち with an earthquake) and likely to be broken down sooner or later. If the
sentence were "...この家ですら壊れてしまうだろう," option c would fit.

"ようものなら" is usually followed by something which accelerates/makes more
probable the foreseeable/expected future.
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2022-08-19T21:36:58.323
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answer
This なる is similar in function to that in `[V dictionary form]-ことになる`.

When you say 地震が起こるとなったら, you are not putting yourself in a situation in which
the earthquake has already happened. For that, you would simply say 地震が起こったら.
Rather, you are supposing a situation in which it has been just determined
that the earthquake will happen, or you have just learned it. The house would
be still standing.
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2022-08-19T21:53:40.037
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answer