What does the conjugations of んだ do?

Question

I have recently reached the の particle in my grammar book and it really confused me.

Question #1: When we attach の to a verb clause like this,

毎日勉強するの大変

does it mean I'm talking about the act of studying everyday and that it classifies as hard, and if so, can this form only be used for dictionary form verbs?

Question #2 : When I'm using の for explanation. This sentence was present in my book:

その人が買うんじゃなかったの?

First of all, why is there another の at the end of the sentence? Isn't it already conjugated as ん? The main question here is: what is the difference between that sentence and this one?

それ人が買ったんじゃない?

If I got it wrong and they're not different could someone tell me what they mean? I know I'm asking a lot and I'm really sorry for that, but both questions got me confused. I'd appreciate any help.

Answer 1

To answer Question #1:

I think there's a mistake in that sentence. It seems like you'd want to say

毎日勉強するのが大変

which would be translated as

Studying everyday is hard.

In this case the の is turning the clause 毎日勉強する into a nominal so it can be used like a noun. "Studying every day".

To answer Question #2:

I think you understand the のだ explaining form very well. And, it would make sense to find it strange to see んじゃなかった and の in the same sentence but the fact is that the の is performing a different function each time it's used.

~ んじゃなない

is a common expression used to emphasize something that is clear to the speaker or seeks agreement from the listener in the form of a rhetorical question. So

その人が買うんじゃなかった?

becomes

Wasn't that person going to buy (it)?

Adding a の at the end more explicitly asks for an explanation from the hearer instead of just a yes or a no.

The sentence you made is definitely correct but has sort of a tense switch

その人が買ったんじゃない?

Didn't that person buy (it)?

The original sentence (from the book) makes it seem like someone was planning on buying something but the speaker thinks that person might not have and they would like an explanation. Your sentence makes it seem like the speaker thought that the person had already bought something but just got information to make him think it didn't happen after all.

Hope that answers your question.