What is the difference between んじゃない and んだ?

Question

…あの犯行予告、俺がやったんじゃないんだけどな。
I wasn't the one who issued that threat, but no matter. (English subs)

What's the difference between んじゃない and んだ as they're used here?

One would ordinarily expect んじゃない to be the negative of んだ. If that were true, then the above sentence would be invoking the explanation modality twice. But it seems that instead んじゃない has an entirely unrelated meaning.

I'm looking for their definitions in relation to each other (to the extent that they can be related). And how does this difference enable them to be used together as in the above sentence?

Answer 1

のだ at the end of a sentence is usually what you called “explanation modality”. (whether it's explanation or not.)

のではない is often used to make a partial negative statement. This may be a over-simplification, but I can't think of a good explanation of its function. The の works like a nominalizer or a quote mark.

If you say 俺はやらなかった, it means I didn't do it.
If you say 俺がやったんじゃない, it means the thing happened, but it's not done by me.

It may be worth mentioning that, both のではない and のだ can sometimes be used to correct an existing proposition. e.g.

俺がやったんじゃない。あいつがやったんだ。

Sometimes you want to empathize “you are thinking wrong”, so you use both のではない (to correct the proposition before it) and のだ (to correct the listener's thought).

俺が やった んじゃない んだ

Although I don't think the last んだ means the same as the んだけどな in your question, it's still a modality のだ, because without んだ it's still a valid sentence.

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It seems that you can read Japanese tutorials, so I added some Japanese resources.

庭{には}三郎{さぶろう} mentioned のではない in his book 日本語文法概説{にほんごぶんぽうがいせつ} 43.1.3　部分否定:

　「～のではない／わけではない」は、「部分否定」とは違いますが、全面的な否定ではない、という意味では近い用法です。

１　資料を全部見たの／わけ　ではない。
２　この事典を調べたのではない。
    cf.　この事典は調べなかった。

例１では、部分否定になっていますが、例２では他の何かと対比しているように感じます。

　複文で理由などを表す節があると、「～のではない」の否定はその節を焦点 とします。

金が欲しいから行ったのではない。

「行った」のですが、その理由は「金が欲しいから」ではない、ということになります。

He says のではない or わけではない is not the same as partial negation but similar to it. It may sound like the part being negated contrasts with something else. It can also be used to negate reason or purpose clauses.

中上級を教える人のための日本語文法ハンドブック has a more detailed explanation at p.301

なお、「のではない」のあとには否定した要素に対応する「正解」[...]が続くのが普通です。

It says のではない is usually followed by a correction.

工藤{くどう}真由美{まゆみ} 's ～のではないの意味と機能 is a more exhaustive study. After reading it, you will know why I say “partial negation” is over-simplification. I quickly went through it. Its explanation might be different from mine (e.g. whether のではない is the negative form of the modality のだ), but it definitely contains valuable information.

Answer 2

Comment

In my experience, once you get to this level it is sometimes easier to understand these intuitively because you can understand a lot from the context and the way the person says the phrase.

Breaking them down can get quite complicated because grammatically there is a lot going on: I have given two ways to analyse 〜たんじゃない but I think they both have merit.

Analysis

My take on

「…あの犯行予告、俺がやったんじゃないんだけどな。」
"I wasn't the one who issued that threat, but no matter."

is:

「・・・」

ー＞ Spoken Japanese, might even contain colloquialisms that seem contradictory just as they do in English ("I haven't eaten none of them.")

"but no matter..."

ー＞ Spoken English also leaves some things unsaid and that seems to be case here. I have not seen the film so I don't know what happens next or before but I guess you will understand from the context.

あの犯行予告、

ー＞ The topic of conversation. あの indicates both parties know what they are talking about.

俺がやった

ー＞ I did it

俺がやったん

ー＞　Something that I did. ん＝の which is a nominaliser like こと. Generally used for things closer to speaker.

じゃない

ー＞ Negates what comes before it ie: 俺がやったんじゃない= Not something that I did.

〜たんじゃない　

ー＞　result of putting (やっ)た＋ん＋じゃない together:

In Japanese: 〜たんじゃない＝〜のではない＝わけではない＝〜というわけではない
In English: ＝"it is not that ~ but rather that," ; only the ~part of the statement is negated

んだ

ー＞　Means same as のです, the sentence ending used for explaining something, and also often to show emotional importance of what comes before it. (So for example, you would tell a doctor: 頭が痛いんです because you want to communicate that it matters.)

けど

ー＞ Equivalent to "but" (like が) and in spoken Japanese the following phrase is often left unsaid because it is obvious from context. It is performing a similar function to the "but, no matter..." in the English translation.　(Or to take another example; 「先生、頭が痛いんですが・・」＝ "Doctor,　my head really hurts [, please can you give me something for it?]")

な

ー＞ Means same as ね、〜 "know what I mean?". な is a harsher more masculine version. It is often used to emphasise the importance of what comes before it and to seek empathy from the listener.

If you put all these together you end up with something close to the translation.

References

I expect you already have references for all the basic components but possibly not 〜たんじゃない because it colloquial? Since writing this I noticed it is covered in the detail of item 3 in 新完全マスターN３、文法, p38.

Understanding this kind of colloquialism is tested at JLPT N1 (聴解). If you want to wok on this you could try one the text books for that exam.

Answer 3

んだ According to my lecturer's explanation, many sources refer to んだ as explanation modality, and it is true many times, you can come across んだ when the sppeaker is giving explanation. However according to his explanation the real function of んだ is to soften up the question and statement.

I myself also try to deepen my skills regarding this topic, I just wanted to add my experience about it.