Working with んだ

Would there be any difference between 学⽣じゃないんだ and 学⽣なんじゃない ?
Also, why is there a need to put な in 学⽣なんだ, 学⽣なんじゃない, 学⽣なんだった or
学⽣なんじゃなかった? Is it because without な it gives the meaning of a possessive?

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学生じゃないんだ is like “Oh (I get it), so (you’re) not a student.” For example you might ask someone what their department is at Uni, and they respond that they actually work for a company (and the obvious implication is that they’re not a student. Perhaps they looked young and you’re surprised, so you reply “あ!学生じゃないんだ”. “Oh right, so you’re not a student then.”

学生なんじゃない is more akin to “Aren’t they/you/etc a student?.” In an example sentence, your friend mentions their son is at work and won’t be back until late. In surprise, you respond "学生なんじゃない?”, “Isn’t your son a student?”, to which they might respond ”学生ですけど、バイトしてる” The じゃない here is like the negative English “Aren’t/isn’t” being used as a question. “Aren’t you going out tonight?” “Isn’t that Mr Rogers?” and so forth.

To answer your second question the なんだ is used as an affirmative, so similar to how I explained in the first part, for 学⽣なんだ the meaning is something like “Oh so you’re a student”, like you understand that now (after perhaps thinking something different previously). A very common example where you’ll hear this なんだ is in the phrase そうなんだ, which means “Oh I see (is that right? I thought otherwise/I didn’t know that)”.

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Because grammar, is why. I’m sure someone can give the full etymological reason, but the short answer is that you’re required to use な between nouns or な-adjecives and んだ.

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The な of な adjectives and the な necessary after nouns when connecting them to things, comes from だ, I’m pretty sure. It’s the “connecting to stuff” form of だ.

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https://www.kokugobunpou.com/助動詞/だ/

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It’s entirely possible that that’s why this requirement came about, but we’d need someone with etymological knowledge to confirm. (I miss you @Carvs!)

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