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




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


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?

2 Answers 2


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.

  • I think this connection to ~ことになる is indeed the part I was missing here. In fact, it's right in my textbook's definition of ~になったら, namely 「~ということになった場合」. I guess I confused that with ~ようになる.
    – L. F.
    Aug 20, 2022 at 0:36

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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