なのだ

Compound of particle な (na, attributive form (連体形) of auxiliary verb だ (da) or copula particle used after the adjective verbs (形容動詞)), の (no, particle to nominalize) and だ (da, auxiliary verb of assertion).

なのだ • (nanoda) 

1. used to indicate a strong assertion
   • 1857-1863, 梅亭金鵞 (Baitei Kinga), 滑稽本 [Kokkeibon]:七偏人 [Shichihenjin], volume 3:

     半(なま)可(ぎ)通(き)の半(なま)酔(ゑひ)だから、此(こ)方(ちら)の誂(あつら)へ向(むき)なのだ

     Namagiki no namaei da kara, kochira no atsuraemuki na no da

     He's a drunken smatterer, so he's perfect for us.

• なんだ (nanda, more informal form)

• なの (na no, feminine form)
• なのです (na no desu, polite form)
• なので (na no de, used as resultative conjunction)
• なのに (na no ni, used as contrastive conjunction)
• のだ (no da)