Hello,
I’ve got a possibly very silly grammar question, but a cursory google search didn’t yield anything helpful, so I’m turning to you, fellow Japanese learners.
I have some confusion regarding んだ and its conjugations. Specifically, what is the difference between adding んだto different conjugations, and conjugating the んだ itself? Like, for example 医者だったんだ and 医者なんだった, or 遊ぶんじゃない and 遊ばないんだ。
Any help is much appreciated. Thanks in advance!
It comes from のだ → for me this んだ adds emotion to my Japaneses sentences.
医者だったんだ。→ “Oh so you’re a doctor.”
医者だった。→ .“You’re a doctor.”
遊ぶんじゃない → stop playing/don’t play/don’t fool around!
遊ばないんだ → I won’t play!
(I don’t know how to explain it like how a textbook would explain it to you and please correct me if I’m wrong.)
(I’m sure you can get a more explicit answer from someone else, but…)
– sometimes it just makes a statement more emphatic // emotional
– sometimes it is used to “indirectly express desires”
I think that first case is relatively easy to pick up on – really, I think if you just see it once, you get the feeling. Pay attention in anime or j-dramas for it ^^
As for the second one, I recall some situation like this from my Japanese class a few years ago:
Say two people are sitting together in a room and a window is open. If person A says 寒いだ～ without such emphatic markers, it’s simply acknowledging that it’s cold. On the other hand, if emphatic markers are used さむいんだ（わ・ね・な・じゃんetc), the feeling is more along the lines of “It’s cold (so maybe we could close the window?”.
To take a scene from real life (that I guess might have been caused by something else also)… a Japanese friend had lent me a book and we were walking to my dorm after lunch so that I could return it to him. As we got to my door I commented, ちょっと汚いんだけど。。 (It’s a little messy…) – to me it felt like a commonplace thing to say… but he actually stopped in his tracks a few feet away from the door, didn’t come inside, and waited (like 5 seconds) for me to fetch the book and come out. I hadn’t noticed that he had stopped because I was only inside so briefly, and laughed when I saw him waiting, asking why he didn’t come inside (as we were quite good friends). He responded that he thought I didn’t want him to/that it would be better not to enter (presumably because of the んだけど there)
Hopefully that’s clearer than mud :;^^
This one, I believe, actually means “You play, don’t you ?”, the じゃない doesn’t actually mean a negation, it’s more of a “isn’t it ?” kind of thing.
@Inuzushi @Neferata Thanks for your replies! However I don’t think that’s quite what I was looking for. Sorry if it wasn’t clear. 
I understand the use of の as a sentence ender to imply a request for an explenation or to convey an explenatory feel. I also understand the addition of だ to give it emphasis or to make the sentence less feminine (hence のだ or んだ). My confusion is regarding the other conjugations and the difference between adding んだ at the end, after the sentence has been conjugated, and conjugating the sentence after the んだ.
So basically
X＋じゃない＋（の）だ as opposed to X＋の＋じゃない
for example
医者じゃないんだ vs 医者なんじゃない
X＋だった＋（の）だ as opposed to X＋の＋だった
for example
医者だったんだ vs 医者なんだった
And
X＋じゃなかった＋（の）だ as opposed to X＋の＋じゃなかった
for example
医者じゃなかったんだ vs 医者なんじゃなかった.
Tae Kim’s grammar guide says that the なんだった and なんじゃなかった is almost never used but I don’t understand the difference between them and だったんだ and じゃなかったんだ.
@Deatrathias ah yes! That does sound familiar. Thank you.
I’m pretty sure of this one. It comes up a lot in animes.
Edit: Your version works too! 遊ぶんじゃない is “You are going to play aren’t you?”
医者じゃない = Not a doctor
医者なんじゃない = A doctor right?
医者じゃなかったんだ = Not a doctor/Wasn’t a doctor
For 医者なんじゃなかった I’ve probably never heard of this one.
For 遊ぶんじゃない can be interpreted as @Deatrathias’s version too. 遊んでいるじゃない means “you’re playing aren’t you?”
You can even say
医者じゃないんじゃない Not a doctor, right ?
Yep that sounds natural.
Uh…okay guys. That’s…still not what I’m asking. But thanks anyway. I appreciate your effort. 
I must not be wording my question very clearly.
This is an explanation of んじゃない from a native.
If you’re asking about the difference between なんだった and だったんだ, besides the fact that なんだった is rarely used, I don’t really know if there is one.
There’s more info on Kara, Node, Noda
Yes, that’s pretty much what I meant. Okay, so no obvious difference in meaning then? Cool, I can work with that.
Thank you.
Also, thanks for the link! I had a read through. Though it didn’t seem to address だったんだ vs なんだった (and the other forms) specifically, it’s still pretty informative. 