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 the subject of a sentence
literary in modern Japanese; usu. written as ヶ in place names
2. indicates possession
Conjunction
3. but, however, (and) yet, though, although, while
4. and
5. used after an introductory remark or explanation