の as an explanatory particle

When do I use の as an explanatory particle? Say if someone asked me “その鞄誰の?” , would it not be apt to reply with "僕の” instead of “僕なんだ” ? Also(unrelated to this), why is it correct to use
んですか but not んだ in questions?

That’s fine. It’s better than 僕なんだ in this case, although people will understand anyway.

I’m not sure I follow what you mean. Both can be used to ask a question (first one is just more formal).

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A textbook might disagree and say that だ is declarative(?) and therefore can’t be used to ask a question. Or something like that. Even if people do it in real life.

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In your example the の is playing a simple possessive role, you’re basically filling in:

俺の(鞄)

Normally when people refer to the explanatory の, they mean something slightly different, using the の particle at the end of a sentence to show they are explaining the reason for something. For example:

寒いの・寒いんだ = it’s cold (and that’s the reason for something).

This is a の which you can attach to a full complete clause in order to give the clause an explanatory feeling. This の is basically filling in the role of a generic noun, therefore turning the sentence into a noun. For example, we can take a clause 今は忙しい, and add んだ to get 今は忙しいんだ. This wraps the first sentence (now (I am) busy), into another one: (xが) 今は忙しいん . So now we need to question, what is x? In this case x is the reason for something, probably why I can’t do something in this case. A good natural English translation is often “the thing”, so we get “The thing is, I’m busy now”.

When doing this to a noun, we need to take the full complete sentence, eg: 先生だ, turn it into its pre-nominal form. Unlike verbs, nouns nominal and adjectival forms are different. This means we need to turn だ into its pre-nominal (連体形) form: な. After that, we can attach んだ as normal, which is where the なんだ from your post comes from.

So 先生なんだ means (roughly) “The thing is he/she is a teacher”, while 先生の means “(something) is the teacher’s”.

So you were right to think that in your sentence you should use 俺の rather than 俺なんだ, because you are trying to form a posessive sentence, but if you really wanted an explanatory sentence (because it’s me), you would want to say 俺なんだ.

Edit: this is pretty long and technical, if you have trouble with some of the basic concepts (but are interested), I would recommend checking out Cure Dolly’s structure series, her voice and weird robot thing take a bit of getting used to, but the grammar explanations are very good and logical.

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Yes, I was taught that だ cannot be used in a question.

です often is just there to make things polite, so it can behave differently from だ, which obeys stricter rules for where it can appear.

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