Particle - のだ - the reason is
This is used when the speaker is explaining something, or asking for an explanation about something. It can be placed directly after a verb or i-adjective. For example: 私は怖いのだ (The reason is I am afraid). When using it after a noun or na-adjective, な is added in between: これは猫なのだ (The reason is that this is a cat).
In conversations のだ is often replaced by んだ. In questions and causal feminine speech its shortened down to just の: これは猫なのか ( Is it that this is a cat? ).
The だ in this particle can be conjugated just as the Informal forms of です. For example: 私は怖いんだった (The reason was because i was afraid).