Why does Bunpro call ないか incorrect here?

Question

It is recommended to use the (いき)ませんか version, as (いか)ないか would be a bit too casual to use with someone that you are not very close with (and is also technically not correct Japanese).

Given ませんか is just ませぬか, with か typically following attributive conjugation in Classical, why would what is equivalent to いかぬか be incorrect in modern Japanese?

Answer 1

I agree with naruto that this is not incorrect, but is inappropriately blunt. We can understand this without reference to historical Japanese, and thinking about language in general terms. Cross-culturally, formal language tends to put more emphasis on the recorded semantics of the words themselves; informal spoken language tends to put more emphasis on intonation (and informal written language will often try to convey that via eye dialect).

ません is a formal negation; it makes sense to expect か in order to distinguish a question from a statement. (Of course, it is still left to the listener to parse the negative question as implicitly an invitation.)

ない is informal; as such, it is expected that the listener will decide whether this is a statement or a question by intonation. Adding か, therefore, carries the implication that specific, heavy emphasis on the fact that it's a question is somehow needed. It will come across blunt or pushy because of the implication that the listener might otherwise ignore the question.

Answer 2

It's true that you should avoid using ～ないか in ordinary conversation. However, this is not because it's technically incorrect but because it's overly blunt.

Grammatically speaking, the following sentences are all correct, but practical choices in daily conversation are only 1 and 2.

1. 映画に行きませんか？

   • Standard polite invitation.

2. 映画(に)行かない？

   • Standard informal invitation. It's a casual suggestion among friends or family members.

3. 映画に行かないか？

   • This is not just informal; it's fairly blunt. It's naturally used by a stereotypical stubborn father or reticent tough guy in fiction. In reality, even my closest friends have never spoken to me like this, but I believe this is a perfectly correct sentence in terms of grammar.

4. 映画に行きませぬか？

   • This is correct as a polite way of speaking like a samurai who has time-slipped into the modern era or a stereotypical knight reincarnated from an isekai. Ordinary people shouldn't use this style in the real world.

5. 映画に行かぬか？

   • A plain (non-polite) version of 4. If such a samurai or knight became your friend, they would speak in this manner.