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!