Is nominalization necessary in 「彼が戻るのか知らないんです。」？

Question

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。

Answer 1

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.

Answer 2

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.)

Answer 3

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.