Hey everybody. I've been lurking these forums for a while, but there's one thing I simply haven't been able to figure out.

With the help of some forum users, I believe I figured out the use of の and のだ in nonconjugated form. This is what I concluded it to be:

When used in a question:
Is the reason for (the thing we both know I am refering to) that x?
When not in a question:
The reason for (the thing we both know I am refering to) is that x
Where x can be a verb, sentence, adjective or noun.

Please correct me if I'm wrong on that one. However, when conjugating it to のじゃない or んだった. It meaning seems to be radically different, and I simpy cannot reach a general consensus on it. Could anybody please explain the meaning of conjugated のだ?

Hi!

Have you read this thread (especially this comment) ?
This is not the easiest part of Japanese grammar. I recommend you to get used to the basic usage of the explanatory 「のだ」, then move on to other more basic grammar points and only learn advanced usages of 「のだ」later.