のです, のだ, んです, んだ
Expression, の and ん add emphasis
the expectation is that ..., the reason is that ..., the fact is that ..., the explanation is that ..., it is that ...
1. indicates sentence subject (occasionally object)
2. indicates possessive (esp. in literary expressions)
3. but, however, still, and
after the volitional form of a verb
4. regardless of, whether (or not)