How does state-of-being + んだ differ from んだ + state-of-being

Question

I was reading up on noun-related particles and came across 「の」 as an explanatory particle.

From my understanding, firstly we need to add 「な」 to differentiate it from relationship 「の」 particle (「ジムなのだ」 vs. 「ジムのだ」). Now, adding it to the end of the sentences adds an explanatory tone that makes the sentence sound like "It is like that ...". e.g. 「学生なのだ」

Then we have this table of combinations as shown.

Here is what I think they mean

• 学生なんだ: I am a student [with an explanatory tone]
• 学生じゃないんだ: I am not a student [ditto.]
• 学生だったんだ: I was a student [ditto.]
• 学生じゃなかったんだ: I wasn't a student [ditto.]

The main problem is I don't understand the second table and this is what I guess are the translations:

• 学生なんだ: I am a student [with an explanatory tone]
• 学生なんじゃない: I am a student, am I not?
• 学生なんだった: I was a student, wasn't I?
• 学生なんじゃなかった: I wasn't a student, was I?

Can someone explain what this particle actually means and what these two tables actually mean?

Answer 1

The tables you shared explain things like differences of don't seem to V and seem not to V or don't try to V and try not to V. They sometimes cause drastic meaning change.

Checking that のだ and なのだ cause strong affirmation, or emphasis, let's start with negation.

Negation

Think about the below negation patterns of "He is a millionaire".

(1) 彼はお金もちじゃないんだ

(2) 彼はお金もちなんじゃない

(1) is simple and easier. The affirmation(なのだ) of negation clause. This just emphasizes the negation clause. It sounds like "He is surely|actually not a millionaire." If the talker of this line was a gold digger, we could get the 'disappointed' feeling.

(2) is difficult. It has 2 major meanings. This is the negation of affirmation(なのだ). Let me show more examples to explain this case.

(2a) 彼はお金もちなんじゃないかと思っている

(2a) I'm thinking he may be a millionaire.

As you may be surprised, the negative meaning disappeared completely in this case. You could get this as the negation of emphasized affirmation(なのだ) becomes 'guessing'!! なのだ is also used to show a confidence to tell a fact. We could understand this form as negating the confidence.

(2b) 彼はお金もちなんじゃない。大富豪だ！

(2b) He is not a millionaire but the billionaire!

In this case, the negation of strong affirmation(なのだ) becomes partial negation like 'partially right but the point is wrong'. This is often used to excuse something.
「わざとやったんじゃないよ」"I didn't do it on purpose." It acknowledges 'I did' but he/she rather wants to say 'not on purpose'.

Past tense

(3) 株を買ったんだ

(4) 株を買うんだった

Let's take examples "株を買う" "buy stocks". It was hard for me to parse out the difference but finally I got the image below, which is the core model to widely understand the difference.

(3) is simple and easier. The affirmation(なのだ) of past tense clause. This just emphasizes the happening in the past while (4) is difficult because (4) has multiple meanings.

(4a) 失敗した。あのとき株を買うんだった

(4a) I missed. I should have bought those stocks.

Along with the image above, we could interpret this sentence like "Once I thought I'm going to buy those stocks". This often shows he/she didn't buy them actually.

(4b) 忘れてた。明日までに株を買うんだった

(4b) It reminded me to buy those stocks by tomorrow.

The fundamental model would be the same as (4a). "Once I thought I'm going to buy those stocks" but in this time it leads to a different meaning like 'forgot but reminded' according to the context.

Negation and Past tense

(5) 彼は学生じゃなかったんだ

(6) 彼は学生なんじゃなかった

(5) is simple. Just emphasizing "He was surely|actually not a student."

In the same way, (6) is 2 ways to interpret. One becomes 'guessing' similar to (2a) meaning like "Once I thought he is a student, so he is a student, right?". The other is 'against the expectation' similar to (4b) like "Once I thought he is a student but actually not".

(7) お酒を飲まなかったんだ

(8) お酒を飲むんじゃなかった

(7) is simple. Just emphasizing "I surely|actually didn't drink alcohol."

In the same way, (8) is 2 ways to interpret. One is 'regretting' similar to (4a) meaning like "Once I thought I should not drink alcohol but actually did." i.e. "I shouldn't have drunk alcohol". The other becomes 'guessing' and 'reminding' mostly with '?' like "Once I thought I have to drink but I still don't, right?" i.e. "I have to drink alcohol, right?"

The each meaning of the second table you shared (negation of のだ, なのだ) highly depends on the context. Japanese usually uses them separately especially by the intonation.

Answer 2

Similar to the phrases だったからだ and だからだった, the tense and polarity before the ん/の is the tense and polarity of the state or the action, and the tense and polarity after the ん/の is the tense and polarity of it's relevance. Your translations are pretty accurate.

A phrase like 学生だったんじゃない? would be close to "isn't it (that/because) they were students?" and 学生なんじゃなかった would be close to "wasn't it (that/because) they are students?". Note: The explanatory tone is probably too explicit in my English translations.

Putting past tense to the right of んだ, especially in relation to a question or statement about the past, could also show more certainty than if past tense is to the left, as in いいんじゃなかった being more certain than よかったんじゃない.

I may be oversimplifying things, of course. As with most grammar paradigms like this, not every possible combination is equally likely or even sounds natural, and there may be other nuances.