I would like to ask you a question about the usage of んだ. I know that it can be used to explain a situation, but I have noticed it can be used to bring up a topic as well, is it true? I heard the following sentence: 仕事やめたんだ In this case I do not think んだ is used to explain a situation because the speakers were talking about a totally different topic. Also, what is the difference between 仕事やめたんだ and 仕事やめた when you want to bring up a topic?
Thank you in advance!
んだ/のだ is used give a supplemental explanation (補足説明{ほそくせつめい})- it is an explanation but not always necessary.
(Xは)仕事{しごと}をやめた。(X depends on the context)
This is describing only the fact that X has quit a job. This is just a statement by the speaker, there is no nuance that the speaker is giving an explanation.
(Xは)仕事{しごと}をやめたんだ。
Using んだ/のだ to '仕事{しごと}をやめた' creates a nuance that the speaker would like to tell the story about why or how X quit the job. This is a big difference from から (e.g. 仕事{しごと}をやめたから) which is normally used to explain a situation.
I always see んだ used with definitive, almost bossy sentences. Like the speaker really wants the listener to know how they feel. So, with respect to what everyone else has said, I would agree. The explanation is given as though a question was asked. Kind of like how a friend will say to you one day out of the blue, "I just quit my job." Naturally, you're going to ask, "Why?" or at least for explanation.
Make more sense?
My dictionary says 「のだ、のです」 is used as the method of representing "explain" and "judgement" including own feeling.
I think 仕事をやめた focus the fact and 仕事をやめたんだ focus more own feeling. In addition, 「んだ、んです」 is a euphonic changes of 「のだ、のです」.
