from Minna no Nihongo: Beginner 2 Translation & Grammatical Notes, lesson 26:
the expression んです is used with only the plain form, for Verbs, i-adjectives and na-adjectives. For Nouns you replace だ with な...
I don't think they use it with the polite form, so there's nothing to worry about.
Oh, it also says that んです is in spoken language, while のです is written...
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from Minna no Nihongo:
from Minna no Nihongo: Beginner 2 Translation & Grammatical Notes, lesson 26:
the expression んです is used with only the plain form, for Verbs, i-adjectives and na-adjectives. For Nouns you replace だ with な...
I don't think they use it with the polite form, so there's nothing to worry about.
Oh, it also says that んです is in spoken language, while のです is written...