のだ is derived from this.
の in [verb]+の+だ・です・が・で etc. is making the verb phrase into a noun. You can substitute the の with a simple noun like もの, if もの doesn't work very well, then こと might work (I think I've seen an instance or two that とき replaces の, or maybe they were something else..).
すきなんだ＝ [na-adjective (すきな like)]＋[noun (の/ん)]＋[verb だ(be)].How is it conjugated if so?
I find it's not necessarily emphasizing, in fact, sometimes it's rather cushioning (すきなんだ) when compared the one without it (すきだ), but it seems always explanatory.