I'm going over the entry for のだ in my grammar dictionary, and I pretty much
understand the meaning, but I'm unsure about casual question forms:

  * んだ (male) / の (female) んだい? (male) / の? (female)

Two examples:

> 「今行くの？」(f)
>
> 「今行くんだい?」(m)

The female form I am okay with. But I have never heard だい before, and I
remember being told that it's very uncommon. I think males might simply say

> 「今行くんだ？」

Am I correct? But I've read that this can sound too interrogative or
accusatory if the intonation is not right, so how would I say it properly?

Finally, I've seen のか, what is the difference? Is it feminine?
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2018-03-28T22:13:19.740
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grammar word-choice
Is it rude to ask a question that ends with だ?
"今行くんだい?" looks highly unnatural to me regardless of gender/situation.
Sentence-end だい in questions appear _mainly by males in fiction_ and _always
with an interrogative_ , e.g., "何故だい?" or "誰だい?" (This い is a [sentence-end
particle](https://dictionary.goo.ne.jp/jn/9100/meaning/m0u/). You can say
"行くか?", "行くのか?" and "誰だ?" but usually not "行くだ?" or "誰か?" in conversation,
right?)

"今行くんだ?" is a natural and casual way of expressing surprise and saying "(Oh,)
you're going now?" It's not particularly masculine in real-world conversation.
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From [Tae Kim's grammar
guide](http://guidetojapanese.org/learn/grammar/sentence_ending):

> かい and だい are strongly masculine sentence endings for asking questions. かい
> is used for yes/no questions while だい is used for open-ended questions.

I believe that guide covers everything you mentioned in your question.
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It is not necessarily rude to ask a question that ends in んだ or のだ (Both are
the same, by the way). And, neither are gendered. The reason ん/のだ can sound
interrogative is because it carries an air of presumption about what you're
asking.

> 今行くの？ = Are you going now?

> 今行くんだ？ = So, you're going now?

Both are functionally the same, but the former is asking, up front, if the
other person is going or not, while the second already _assumes_ the person
_is_ leaving, and is merely checking.

As for のか, it is not gendered, either. In fact, のか can be pretty different
from just か/ですか/の. I would read [this
thread](https://japanese.stackexchange.com/questions/18613/what-is-the-
meaning-of-%E3%81%AE%E3%81%8B-and-how-does-it-differ-from-%E3%81%8B) here on
it. But, assuming it is being used in a non wh-question, _and_ does not appear
in the middle of sentence (aka it is being used as a sentence-ending
particle), the best way to describe it, I think, is to think of it as
expressing doubt, while ですか or の are more neutral.

> 本当に面白いの？ = is [it] actually funny? (neutral)

> 本当に面白いのか? = is [it] actually _that_ funny? (doubting)
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