Do we need to nominalize 彼が戻る with の to create an interrogative clause? Is B not enough?

A: 彼が戻るのか知らないんです。

B: 彼が戻るか知らないんです。

  • のか isn't quite using the nominalizer の; it's related to のだ.
    – Angelos
    May 25, 2016 at 12:04
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    @Nothingatall: Please explain it more detailed. If so, it should be のだか, right? May 25, 2016 at 12:09
  • I don't truly feel qualified to explain, but I'm sure someone will come along who does.
    – Angelos
    May 25, 2016 at 12:26
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    But you could say のだ is a special use of the nominalizer の. @YasashiiEirian, だ+か can't appear together in a main clause, but in subordinate interrogative clauses だ can optionally be retained, so you can have both 誰だか知らない and 誰か知らない.
    – user1478
    May 25, 2016 at 16:46
  • @snailplane That's why I said it isn't quite the nominalizer, emphasis on quite. In any case this is an embedded question.
    – Angelos
    May 28, 2016 at 9:02

3 Answers 3


This の is not the nominalizing の, it's the explanatory の, as in 「ちょっと話{はな}したいことがあるのですが。」"There is something I would like to talk to you about (explanatory tone)." When used in a question, it takes the reverse nuance of seeking an explanation, as in 「どこにいたの?」"Where were you? (seeking explanation)."

In your example sentences, the の is technically unnecessary, as both A and B are perfectly grammatical, however the nuance and tone sound very different.

Sentence B: 「彼{かれ}が戻る{もどる}か知{し}らないんです。」sounds very literal and matter-of-fact, as in "I don't know whether he'll return."

Sentence A: 「彼が戻るのか知らないんです。」makes the speaker sound a little more curious about the fate of this man (since the speaker is now seeking an explanation), as in "I don't know whether he'll return (but am curious to find out)."

Finally, it's interesting to note that both sentences also feature 〜んです at the end, which is a common contraction for 〜のです that is easier to say and sounds softer. This 知らないんです is simply telling the listener that they are trying to explain the range of what they know.


No, you don't need it unless the sentence has どうして or なぜ.

(Incidentally, わからない is better rather than 知らない here.)


What skywalker said is mostly correct, but using の after 戻る makes no sense as that is not the final/controlling verb of the sentence. If you did want to add the explanatory/expecting an answer nuance then you would do it by adding のだ or のか to the end of the sentence.

As for what user4092 said, the correct verb is 知る, but native Japanese speakers will often use 分かる inappropriately in situations where the former is called for so everyone will understand you either way.

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    「native Japanese speakers will often use 分かる inappropriately in situations where the former is called for」 <- In what situation, for example?
    – chocolate
    May 26, 2016 at 4:42
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    Must say, I'm deeply intrigued by the idea that native Japanese speakers will often use 分かる inappropriately. Specifically, what criterion of appropriateness is making so that native speakers are erring in the use of their own language (I don't think that's automatically impossible but... doesn't native usage primarily dictate the rules?)
    – virmaior
    May 26, 2016 at 5:25

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