のです, んです
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 ...
お借りした車の調子が悪いのですが。
が
Particle
1. indicates sentence subject (occasionally object)
2. indicates possessive (esp. in literary expressions)
Conjunction
3. but, however, still, and
after the volitional form of a verb
4. regardless of, whether (or not)